There is no standard variety of Scottish Gaelic; although statements below are about all or most dialects, the north-western dialects (Outer Hebrides, Skye and the Northwest Highlands) are discussed more than others as they represent the majority of speakers.

Gaelic phonology is characterised by:

• a phoneme inventory particularly rich in sonorant coronal phonemes (commonly nine in total)
• a contrasting set of palatalised and non-palatalised consonants
• strong initial word-stress and vowel reduction in unstressed syllables
• The presence of preaspiration of stops in certain contexts
• falling intonation in most types of sentences, including questions
• lenition and extreme sandhi phenomena

Due to the geographic concentration of Gaelic speakers along the western seaboard with its numerous islands, Gaelic dialectologists tend to ascribe each island its own dialect. On the mainland, no clear dialect boundaries have been established to date but the main areas are generally assumed to be Argyllshire, Perthshire, Moidart/Ardnamurchan, Wester Ross and Sutherland.

Descriptions of the language have largely focused on the phonology. Welsh naturalist Edward Lhuyd published the earliest major work on Scottish Gaelic after collecting data in the Scottish Highlands between 1699 and 1700, in particular data on Argyll Gaelic and the now obsolete dialects of north-east Inverness-shire.^[1]

Following a significant gap, the middle to the end of the twentieth century saw a great flurry of dialect studies in particular by Scandinavian scholars, again focussing largely on phonology:

• 1938 Nils Holmer Studies on Argyllshire Gaelic published by the University of Uppsala
• 1937 Carl Borgstrøm The Dialect of Barra published by Norsk Tidsskrift for Sprogvidenskap
• 1940 Carl Borgstrøm The Dialects of the Outer Hebrides published by Norsk Tidsskrift for Sprogvidenskap
• 1941 Carl Borgstrøm The Dialects of Skye and Ross-shire published by the Norwegian University Press
• 1956 Magne Oftedal The Gaelic of Leurbost, Isle of Lewis published by Norsk Tidsskrift for Sprogvidenskap
• 1957 Nils Holmer The Gaelic of Kintyre published by the Dublin Institute for Advanced Studies
• 1962 Nils Holmer The Gaelic of Arran published by the Dublin Institute for Advanced Studies
• 1966 Gordon MacGillFhinnein Gàidhlig Uibhist a Deas ("South Uist Gaelic") published by the Dublin Institute for Advanced Studies
• 1973 Elmar Ternes The Phonemic Analysis of Scottish Gaelic (focussing on Applecross Gaelic) published by the Helmut Buske Verlag
• 1978 Nancy Dorian East Sutherland Gaelic published by the Dublin Institute for Advanced Studies
• 1989 Máirtín Ó Murchú East Perthshire Gaelic published by the Dublin Institute for Advanced Studies

In the period between 1950 and 1963, fieldwork was carried out to document all then remaining Gaelic dialects, culminating in the publication of the five-volume Survey of the Gaelic Dialects of Scotland by the Dublin Institute for Advanced Studies in 1997. The survey collected data from informants as far south as Arran, Cowal, Brig o' Turk, east to Blairgowrie, Braemar and Grantown-on-Spey, north-east to Dunbeath and Portskerra and all areas west of these areas, including St Kilda.

The following is a chart of the monophthong vowel phonemes appearing in Scottish Gaelic:^[2]

All vowel phonemes except for /ɪ/ and /ə/ can be both long (represented with ⟨ː⟩) and short. Phonologically, /a/ behaves both as a front or back vowel depending on the geographical area and vowel length.

Like the closely related languages Modern Irish and Manx, Scottish Gaelic contains what are traditionally referred to as "broad" and "slender" consonants. Historically, Primitive Irish consonants preceding the front vowels /e/ and /i/ developed a [j]-like coarticulation similar to the palatalized consonants found in Russian^[5][6] while the consonants preceding the non-front vowels /a/, /o/ and /u/ developed a velar coarticulation. While Irish distinguishes "broad" (i.e. phonetically velar or velarised consonants) and "slender" (i.e. phonetically palatal or palatalised consonants), in Scottish Gaelic velarisation is only present for /n̪ˠ l̪ˠ rˠ/. This means that consonants marked "broad" by the orthography are, for the most part, simply unmarked, while "slender" consonants are palatal or palatalised. In the modern languages, there is sometimes a stronger contrast from Old Gaelic in the assumed meaning of "broad" and "slender"; the phonetic distinction can be more complex than mere "velarisation"/"palatalisation". For instance, the Gaelic "slender s" is so palatalised that it has become postalveolar [ʃ]. The main exception to this contrast are the labials (/p pʰ m f v/), which have lost their palatalised forms. The only trace of their original palatalisation is a glide found before a back vowel, e.g. beum /peːm/ ('stroke') vs beò /pjɔː/ ('alive'). Celtic linguists traditionally transcribe slender consonants with an apostrophe (or more accurately, a prime) following the consonant (e.g. ⟨m′⟩) and leave broad consonants unmarked.

The unaspirated stops in some dialects (east and south) are voiced (see below), as in Manx and Irish. In the Gaelic of Sutherland and the MacKay Country, this is the case, while in all other areas full voicing is allophonic with regional variation. Voicing additionally occurs in certain environments, such as within breath groups and following homorganic nasals (see below).^{[citation needed]} The variation suggests that the unaspirated stops at the underlying phonological level are voiced, with devoicing an allophonic variant that in some dialects has become the most common realisation. East Perthshire Gaelic reportedly lacks either a voicing or an aspiration distinction and has merged these stops. Irish dialects and Manx also have devoiced unaspirated consonants in certain environments.

Certain consonants (in particular the fricatives [h x ç ɣ ʝ v] and the lenis coronals [l n ɾ ɾʲ]) are rare in initial position except as a result of lenition.

Stress is usually on the first syllable: for example drochaid ('a bridge') [ˈt̪rɔxɪtʲ]. Words where stress falls on another syllable are generally indicated by hyphens: these include certain adverbs such as an-diugh ('today') [əɲˈdʲu] and an-còmhnaidh ('always') [əŋˈgɔ̃ːnɪ]. In loanwords, a long vowel outside the first syllable is also indicative of stress shift, for example buntàta ('potato') [bən̪ˠˈt̪aːht̪ə]. Stress shift may also occur in close compound nouns or nouns with prefixes, though the stress patterns here are less predictable, for instance in beul-aithris ('oral literature') can be both [ˈpial̪ˠaɾʲɪʃ] or [pjal̪ˠˈaɾʲɪʃ].

A distinctive characteristic of Gaelic pronunciation (also present in Scots and Scottish English dialects (cf. girl [ɡɪɾəl] and film [fɪləm]) is the insertion of epenthetic vowels between certain adjacent consonants. This affects orthographic l n r when followed by orthographic b bh ch g gh m mh; and orthographic m followed by l r s ch.

tarbh ('bull') — [t̪ʰaɾav]

Alba ('Scotland') — [al̪ˠapə].

Occasionally, there are irregular occurrences of the epenthetic vowel, for example in Glaschu /kl̪ˠas̪əxu/ ('Glasgow').

There are often wide variations in vowel quality in epenthetic vowels, as illustrated by a map showing the pronunciations of dearbh.^[9]

• Area 1 [tʲɛɾav]
• Area 2 [tʲaɾa(v)] with the [v] appearing in the northwestern region but not the southeastern
• Area 3 [tʲɛɾɛv]
• Area 4 [tʲɛɾʊ] with vocalization of the [v]
• Area 5 [tʲɛɾəv] with reduction of the epenthetic vowel as in Irish

A schwa [ə] at the end of a word is dropped when followed by a word beginning with a vowel. For example:

duine ('a man') — [ˈt̪ɯɲə]

an duine agad ('your man') — [ən̪ˠ ˈt̪ɯɲ akət̪]

Of all the Celtic languages, lexical tones only exist in the dialects of Lewis^[17] and Sutherland^[18] in the extreme north of the Gaelic-speaking area. Phonetically and historically, these resemble the tones of Norway, Sweden and southwestern Denmark; these languages have tonal contours typical for monosyllabic words and those for disyllabic words. In Lewis Gaelic, it is difficult to find minimal pairs. Among the rare examples are: bodh(a) [po.ə] ('underwater rock') vs. bò [poː] ('cow'), and fitheach [fi.əx] ('raven') vs. fiach [fiəx] ('debt'). Another example is the tonal difference between ainm [ɛnɛm] and anam [anam], the latter of which has the tonal contour appropriate to a disyllable. These tonal differences are not to be found in Ireland or elsewhere in the Scottish Gàidhealtachd.^[19] Furthermore, they are disappearing entirely among younger speakers even in Lewis.^[20]