Positions of double bonds of olefins of complex mixtures, such as those of Drosophila cuticular hydrocarbons, have been determined using a simple method combining methoxymercuration-demercuration of extracts and analysis by gas chromatography-mass spectrometry using ammonia for chemical ionization. Chemical similarities between cuticular monoenes of both sexes of Drosophila simulans and males of Drosophila melanogaster are specified: the major isomer is always in position 7.