Condensable molecules generated in the gas phase by chemical reaction can either form new panicles or condense on existing aerosol particles. Experiments were carried out to study the effect of a preexisting aerosol on aerosol dynamics in such systems. Secondary sulfate aerosol was produced by photochemical reactions in the SO2-NOx-propylene system. A new theory which predicts the rate at which new particles of a given size are formed is also presented. Scavenging of molecular clusters by surrounding aerosols can be important, and is incorporated in the theory. Measured rates of new particle formation are compared with predicted rates, taking into account the minimum particle size detected experimentally. Agreement between theory and experiment is within experimental error. Based on the theoretical analysis, a criterion for determining whether or not new particle formation is important in such systems is established.