Fifty-three cosmetic products containing one or more of the ingredients N,N-dimethyloctadecylamine oxide, N,N-dimethyloctadecylamine and N-benzyl-N,N-dimethyloctadecylammonium chloride were analysed for N-nitroso-N-methyloctadecylamine by gas chromatography with detection by a Thermal Energy Analyzer. [1-14C]N-Nitroso-N-methyloctadecylamine was used as an internal standard. Eight of 11 products containing N,N-dimethyloctadecylamine oxide and three of 38 products containing N-benzyl-N,N-dimethyloctadecylammonium chloride were found to contain N-nitroso-N-methyloctadecylamine at levels ranging from 28 to 969 ppb. In photolysis experiments, all of these products exhibited a loss of Thermal Energy Analyzer response for N-nitroso-N-methyloctadecylamine following irradiation by ultraviolet light. In two cases, the presence of N-nitroso-N-methyloctadecylamine was confirmed by combined gas chromatography-mass spectrometry.