AlxGa1-xN-layers are applied in various devices based on group-III-nitride semiconductors. Crystalline quality, composition and morphology need to be controlled by properly adjusting growth processes. Here we report on AlxGa1-xN-growth by low pressure MOVPE on planar surfaces of sapphire with focus on Al-incorporation. The solid Al-composition x(A1) depends critically on growth parameters for constant gas phase compositions. The strongest effect is found for total pressure: Al is efficiently incorporated only at pressures lower than 50 mbar. This can be understood mainly as a result of prereactions in the gas phase involving TMAl and NH3 which has been discussed earlier in a theoretical paper of the Jensen group [T.G. Mihopoulos, V. Gupta, K.F. Jensen, J. Crystal Growth 195 (1998) 733]. In this publication we compare the predictions of this work with our data and implications for growth optimization. (c) 2006 Published by Elsevier B.V.