In chemistry, a lyonium ion is the cation derived by the protonation of a solvent molecule.^[1] For example, a hydronium ion is formed by the protonation of water, and CH₃OH+2 is the cation formed by the protonation of methanol.

Its counterpart is a lyate ion, the anion formed by the deprotonation of a solvent molecule.

Lyonium and lyate ions, resulting from molecular autoionization, contribute to the molar conductivity of protolytic solvents.

• Lyate ion, a deprotonated solvent molecule
• Onium ion, a protonated molecule more generally
• Ion transport number
• Ionic atmosphere