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Sampling and analysis methods for unregulated exhaust constituents are discussed. Emission results for more than fifteen exhaust constituents from both gasoline- and diesel-powered automobiles are presented. It is shown that the catalytic converter substantially lowers the emission rates of aldehydes, benzene, benzo(a)-pyrene, hydrogen cyanide, and nitrogen dioxide. However, under certain rich-malfunction conditions, small increases in hydrogen sulfide, carbonyl sulfide, hydrogen cyanide, and ammonia occur. Particulate emissions are the primary concern for diesels since other unregulated emissions occur at the same low levels as from gasoline-powered vehicles. It is concluded that although steady improvements in chemical analysis technology have led to the detection of more and more minor impurities in exhaust, none of these substances are emitted at concentrations that can be considered dangerous.