A model mechanism to describe the oxidation of α-pinene and β-pinene has been developed based on the chemistry of lower alkenes and smog chamber studies of gas and aerosol phase products of monoterpene oxidation described in the literature. The mechanism is designed to replace single-step aerosol yield or fractional aerosol coefficient approaches currently used in numerical atmospheric chemistry transport models. The mechanism predicts the concentrations of condensable products which are partitioned between gas and aerosol phases based on the absorption approach of Odum et al. <1996> and Hoffmann et al. <1997> but uses a six-product rather than a two-product model. Model-predicted aerosol concentrations and yields are similar to those observed in the smog chamber, and the timing of aerosol formation is in reasonable accord with that observed. The monoterpene mechanism is also used to develop aerosol isopleths based on NOx and α-pinene concentrations and temperatures representative of those found in the atmosphere. ¿ 1999 American Geophysical Union <