The growth of GaN by HVPE was analysed by means of a thermodynamical and kinetic study. The thermodynamical constants of the reactions involved in the GaN growth and the partition functions of the molecules used in the kinetic study were calculated. The kinetic coefficients and the activation energies of the reactions were tabulated. Good agreement was obtained between the model and the experimental results. The influence of the reactor geometry and of the parasitic nucleation on the glass walls of the reactor was demonstrated. After analysing the physical phenomena which might take place in the vapour phase, we concluded that the vapour phase was not homogeneous. The reaction of formation of GaCl3 from HCl and GaCl appears to be incomplete at the GaCl inlet, and almost complete over the substrate. Both effects explain the difficulty of growing GaN layers without extraneous deposit.