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 in a turbulent coaxial jet. In presented experiments, the influence of velocities of gases and nozzle shape on particle size distributions has been studied. Experimental distributions have been evaluated by in situ light-scattering measurements. Theoretical particle size distributions resulting from a simple model are compared to experimental measurements. Results show that particles do not seem to grow only by agglomeration and mixing near the nozzle may strongly influence final particle size distributions. In fact, the development of mixing layers near the nozzle appears to have little effect on particle size distributions, so that mesomixing could be the major phenomenon.