Data on smog-forming reactivities of exhaust aldehydes are scarce. Such data were obtained by irradiating in a smog chamber typical exhaust aldehydes, individually and in exhaustlike mixtures. Observed reactivity manifestations were rate of nitrogen dioxide formation (rNO2) and yields of oxidant, peroxyacetyl nitrate, and formaldehyde. Results showed that exhaust aldehydes vary widely in reactivity. Formaldehyde showed high rNO2 reactivity but low oxidant yield; benzaldehyde, m-, and p-tolualdehyde were totally unreactive when irradiated individually; o-tolualdehyde was reactive. In mixtures, bensaldehyde manifested reactivity in terms of peroxybenzoyl nitrate yield. Exhaust aldehydes as a group are judged to be as reactive as the exhaust olefin group. The importance of the aldehyde contribution to exhaust reactivity and methods for assessing such contribution are discussed. Work done under an agreement with the Air Pollution Control Office of the Environmental Protection Agency (formerly National Air Pollution Control Administration, U.S. Department of Health, Education, and Welfare).