The impact on growth efficiency and growth profiles of gas phase nucleation phenomena during MOVPE of GaN is investigated by a series of growth runs in a production scale Planetary Reactor (R). Parameter variations are carried out to distinguish between the effects of pressure, metalorganic precursor partial pressure, residence time and thermal ambient. The results are used to validate and refine a computational model describing gas phase reaction kinetics and nucleation dynamics. Modelling is used to elucidate critical routes of depletion by gas phase nucleation. The control of these processes has led the way to uniform layer growth over 6 inch wafers at high pressure and high growth rates in a larger next generation Planetary Reactor (R). (C) 2010 Elsevier B.V. All rights reserved.