In the study, presented in this paper, the growth of GaN layers by Hydride vapour phase epitaxy was investigated both experimentally and numerically. Modelling the flows in the reactor, combined with basic chemistry, gives a good approximation to the actual growth experiments. It was found that small changes in the reactor geometry, e.g. the inlet of GaCl, have a large effect on the growth rate as well as on the uniformity of the growth. (C) 2004 Elsevier B.V. All rights reserved.