This paper deals with an experimental study and a modelling of the formation of particles in a reactive gaseous jet. Particles result from the gas-phase hydrolysis of tin tetrachloride or titanium tetrachloride in a turbulent coaxial jet. In our experiments, the influence of velocities of gases on particle size distributions has been more particularly studied. Experimental particle size distributions have been evaluated by in situ light-scattering measurements. Results are compared to theoretical particle size distributions obtained with a simple model taking into account mixing of reagents and agglomeration of particles. It appears that growth of particles seems to be controlled by agglomeration for TiCl₄/H₂O jets whereas another growth phenomenon may also be involved for SnCl₄/H₂O jets. In this latter case, mixing near the nozzle may have a strong influence on final particle size distributions.