The autoignition temperature or self-ignition temperature, often called spontaneous ignition temperature or minimum ignition temperature (or shortly ignition temperature) and formerly also known as kindling point, of a substance is the lowest temperature in which it spontaneously ignites in a normal atmosphere without an external source of ignition, such as a flame or spark.^[1] This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical ignites decreases as the pressure is increased.

• Substances which spontaneously ignite in a normal atmosphere at naturally ambient temperatures are termed pyrophoric.

Autoignition temperatures of liquid chemicals are typically measured using a 500-millilitre (18 imp fl oz; 17 US fl oz) flask placed in a temperature-controlled oven in accordance with the procedure described in ASTM E659.^[2]

When measured for plastics, autoignition temperature can be also measured under elevated pressure and at 100% oxygen concentration. The resulting value is used as a predictor of viability for high-oxygen service. The main testing standard for this is ASTM G72.^[3]

The time Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "http://localhost:6011/en.wikipedia.org/v1/":): {\displaystyle t_\text{ig}} it takes for a material to reach its autoignition temperature ${\displaystyle T_{\text{ig}}}$ when exposed to a heat flux ${\displaystyle q''}$ is given by the following equation:^[4]

${\displaystyle t_{\text{ig}}={\frac {\pi }{4}}k\rho c\left[{\frac {T_{\text{ig}}-T_{0}}{q''}}\right]^{2},}$

where k = thermal conductivity, ρ = density, and c = specific heat capacity of the material of interest, ${\displaystyle T_{0}}$ is the initial temperature of the material (or the temperature of the bulk material).

Temperatures vary widely in the literature and should only be used as estimates. Factors that may cause variation include partial pressure of oxygen, altitude, humidity, and amount of time required for ignition. Generally the autoignition temperature for hydrocarbon/air mixtures decreases with increasing molecular mass and increasing chain length. The autoignition temperature is also higher for branched-chain hydrocarbons than for straight-chain hydrocarbons.^[5]

• Fire point
• Flash point
• Gas burner (for flame temperatures, combustion heat energy values and ignition temperatures)
• Spontaneous combustion