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Twenty-three hydrocarbons were measured in the exhaust gases from a Volvo passenger car fitted with a 2.3 litre gasoline engine. Measurements were made in the absence of a catalyst, and in the presence of a fresh and an aged three-way catalyst, with particular attention being paid to the emission of benzene and other light aromatic compounds. Loss of catalytic activity through ageing led to an increase in hydrocarbons of ∼ 200% from 0.22 to 0.62 g/mile. Loss of activity was most evident for certain compounds notably alkanes (paraffins) although large increases in aromatic emissions were also apparent; catalytic control of ethyne (acetylene) was, however, completely maintained by the aged catalyst. Thus the work reported here demonstrates the selective manner in which a catalyst operates depending upon the chemical structure of hydrocarbons, and how this influences catalyst performance loss via ageing.