The flow pattern of the mixture of reacting substances in a vertical MOCVD reactor was studied. The physical modelling of a non-isothermal laminar flow of mixture of gases in a reaction chamber was performed. Parameters of the model, based on the criteria of similarity conservation, were determined by means of the dimensional analysis. Flow visualization in the model of the reaction chamber was performed using a smoke tracer technique. The effect of the shape of the chamber inlet was analysed, as well as the difference in densities of the used gases on the velocity distribution. The flow visualization experiments proved that the stability of a separating surface determines a flow pattern in the chamber. On the basis of the results yielded of the flow visualization experiments some modifications of the chamber inlet shape and the method of injection in the vertical MOCVD reactor were proposed.