A study is made of the flow, heat and mass transfer, particle formation and deposition, including the SiCl4 and GeCl4 oxidation reactions and the effects of buoyancy, variable properties and tube rotation with application to the modified chemical vapor deposition (MCVD) process. The steady-state three-dimensional governing conservation equations, which include the continuity, species, momentum and energy equations have been solved numerically. The resulting temperature, velocity and species concentration fields permit the determination of the particle formation and particle deposition efficiencies. Of special interest are the effects of the chemical reactions on particle formation and deposition.