In situ line of-sight. mass spectrometry facilitates a quantitative method to determine the current growth rate during molecular beam epitaxy of GaN. For homoepitaxy on GaN templates the growth rate reaches its steady-state value immediately after growth is initiated. On the other hand, heteroepitaxy on 6H-SiC or sapphire is characterized by a reduced growth rate during the initial phase of nucleation leading to a quadratic evolution of film thickness with time. Atomic force microscopy and Rutherford backscattering reveal that this behavior is caused by a reduced sticking coefficient and incomplete wetting of the substrate. Under typical growth conditions the resulting loss in film thickness amounts to 8 +/- 1.5 nm. GaN growth on AlN however follows the Stranski-Krastanow growth mode exhibiting initially a fast linear increase of the growth rate to 2/3 of its final value. After the transition from two- to three-dimensional growth at 1.6 monolayers the growth rate exponentially approaches the steady-state value yielding a total loss in film thickness of 1 nm. (C) 2002 Elsevier Science B.V. All rights reserved.