We present a coupled modelling approach involving mass transport and local thermochemical equilibrium in the gas phase and at the deposition surface of the chemical vapour deposition (CVD) process. The theoretical problems arising from the combination of the two approaches are shown and a solution procedure is proposed. Selected results of thermodynamic and mass transport analysis and of the coupled approach show that, for the deposition of Si1-xGex solid solution at 1300 K, the thermodynamic heterogeneous stability of the reactive gases and thermal diffusion lead to the depletion of germanium in the deposit.