This work describes the formation process of formaldehyde and acetaldehyde in the exhaust of spark-ignited internal combustion engines from oxidation of unburned ethanol, methane or ethane. The last two are formed as intermediate products of gasoline or ethanol combustion. The oxidation reaction rate equilibrium constants for methane and ethane were determined from Gibbs function. Exhaust aldehydes were quantified for engines fuelled by gasoline-ethanol blend. The results demonstrate the influence of ethanol, methane and ethane concentration on formaldehyde and acetaldehyde formation as a function of the exhaust temperature.