Petrol (gasoline) blends for road vehicle use are increasingly being supplemented with alcohols, particularly ethanol or butanol produced by fermentation of plant feedstocks. This paper critically reviews the fate and effects of alcohol–petrol blends in soil and groundwater, taking account of the formulations marketed in the UK and their predicted future use patterns. Because of their partitioning behaviour and high biodegradability, the alcohols are not predicted to migrate extensively in the subsurface nor are they expected to increase migration of hydrocarbon components of the petrol through cosolvency or changes in the physical–chemical properties of non-aqueous phase liquid hydrocarbons. Rapid alcohol biodegradation can result in highly anaerobic subsurface conditions that can lead to a reduced rate of hydrocarbon biodegradation and the consequent development of plumes larger than those resulting from alcohol-free petrol. The evidence shows, however, that releases of alcohol-blended petrol are highly unlikely to result in a significantly increased risk to groundwater resources in the UK. Furthermore, biogenic methane resulting from rapid alcohol biodegradation is unlikely to result in potentially hazardous indoor air concentrations except where a major release of a high concentration alcohol formulation occurs, in which event case-specific assessment would be necessary.