The relative strengths of the sources of atmospheric hydrocarbons in Sydney have been determined by a source reconciliation technique, which involved the detailed characterization of the compositions of the major hydrocarbon sources and about 150 determinations of the concentrations of numerous individual hydrocarbons in the air. The principal sources of atmospheric hydrocarbons are shown to be vehicle exhaust (36 ± 4% w/w of non-methane hydrocarbons), evaporative emissions of petrol (32 ± 4 % w/w) and evaporation of hydrocarbon and other solvents (23 ± 4 % w/w). The reliability of the results is confirmed by an analysis of the sensitivity of the source strength calculations to uncertainties in the source compositions and in the atmospheric concentrations, and to the reactivity of the hydrocarbons. The relative source strengths are compared with an inventory of hydrocarbon emissions in Sydney carried out by the State Pollution Control Commission. The results obtained by the two techniques agree well, except for petrol evaporation for which the source reconciliation method indicates a somewhat higher contribution.