Kinetics of formation and growth of finely disperse aerosol at tungsten carbonyl photolysis in air is studied experimentally. The emphasis is on the effect of coagulation and the interval between termination of the generation and the time of particle detection (t1) on the measured concentration and the particle size distribution. Kinetics of aerosol formation is numerically simulated within the framework of the Smolukhovskii coagulation model. The rate of generation of condensing products, “monomers” F(t), was derived from comparison between the experimental and calculated data. The derived value proves to agree well with the independently measured rate of W(CO)6 photolysis.