AIN deposition experiments from AlCl3/NH3 mixtures were carried out in a hot wall chemical vapour deposition (CVD) reactor. The thickness profiles of the deposited films suggested that both reactions and transport of the gaseous species probably play a key role in the deposition process, so a two-dimensional numerical model was developed to analyse mass transfer inside the reactor. From the available information on the AlCl3/NH3 or BCl3/NH3 chemistry, a chemical scheme was worked out. The homogeneous rate constants were taken from previous works, whereas the analytical expressions of the heterogeneous reaction rates were simply obtained by matching the calculated and experimental deposition rates. Our numerical analysis shows that the heterogeneous reaction of AlCl3 and NH3 is an important step in the deposition process, but the homogeneous reaction between AlCl3 and NH3 to form AlCl3NH3. should also be considered. This analysis also demonstrates that the deposition is controlled by diffusion in some regions inside the reactor. The growth mechanisms of AIN will be further analysed thanks to this modelling, in a second paper.