We report on the use of in-situ optical sensors during the MOVPE of GaN for determining the growth temperatures and the growth rates over the temperature ranges employed for nucleation between 470 and 570degreesC and for high-temperature epilayer growth between 960 and 1100degreesC, respectively. Experiments were carried out in a usual MOVPE growth equipment designed for epitaxy, using two different gas inlet geometries. The influence of temperature and the demands on temperature control in the respective growth steps were assessed comparatively for both geometries. It was found that the precursors' injection geometry strongly influences the activation energy in the low-temperature kinetically controlled nucleation layer step. The onset of the thermodynamically controlled growth regime is also influenced by the injection geometry. (C) 2004 Elsevier B.V. All rights reserved.