This paper deals with the study of the kinetics of photolysis and photonucleation of acetaldehyde (AA) vapor in air and in nitrogen along with physical and chemical characteristics of the formed aerosol particles. Free radicals accompanying photonucleation were identified using the electron spin resonance spin-trapping technique. Numerical modeling of the photonucleation kinetics of AA, based on experimental results, was carried out. Rate of generation of primary products, leading to the nanoparticles, and the aerosol yield were evaluated. Chemical mechanism of stages leading to photonucleation in this system was proposed.