The kinetics of photolysis and photonucleation of benzaldehyde (BA) vapor in the air and in an inert gas is investigated, along with the physical and chemical characteristics of the resulting aerosol particles. Short-lived free radicals that accompany the BA photonucleation process were identified by means of spin trapping. Numerical simulation of BA photonucleation, combined with the proposed chemical mechanism, allowed us to calculate the rate of generation of initial particles and to estimate the aerosol quantum yield.