A mathematical model predicting the particle size distribution of solid dispersed phase obtained as a product of a reaction between gaseous reactants is proposed. The model assumes that the particle formation occurs in different steps: gas phase reaction, particle nucleation, particle growth via surface reaction and coagulation. Integro-differential equations describing the process are set-up and solved by means of the moments method. As a result expressions for the first two moments of the particle size distribution, giving the particle concentration and the mean particle volume, are obtained as a function of time, reactant initial concentration, and nucleation, surface reaction and coagulation kinetic constants. Some characteristic times of the particles formation process are also derived and discussed.