Ethanol (ethyl alcohol) and methanol (methyl alcohol) are two types of alcohol fuels. The use of pure alcohols in internal combustion engines is only possible if the engine is designed or modified for that purpose. However, in their anhydrous or pure forms, they can be mixed with gasoline (petrol) in various ratios for use in unmodified gasoline engines, and with minor modifications can also be used with a higher content of ethanol. Typically, only ethanol is used widely in this manner, particularly since methanol is toxic.

Ethanol fuel mixtures have "E" numbers which describe the percentage of ethanol in the mixture by volume, for example, E85 is 85% anhydrous ethanol and 15% gasoline. Gasoline is the typical fuel mixed with ethanol but there are other fuel additives that can be mixed, such as an ignition improver used in the E95 Swedish blend. Low ethanol blends, from E5 to E25, are also known as gasohol, though internationally the most common use of the term gasohol refers to the E10 blend.