The effect of anneal temperature on the nucleation layer transformation of GaN deposited on sapphire by metalorganic vapour phase epitaxy has been investigated by in situ reflectivity, ex situ transmission electron microscopy and atomic force microscopy. The ripening process is characterized by substantial mass transport by which wurzite material is redeposited on top of the pre-existent nuclei. By analyzing the effects of the nuclei characteristics on the reflectivity signal it was found that the ripening-recrystallization process exhibits a temperature dependent onset and a temperature accelerated transformation rate indicating a rather strongly thermally activated phenomenon. A modeling of the in situ reflectivity data has been used for the interpretation of its behaviour during anneal, which allows the characteristic features of the phenomenon to be identified. The data indicate an activation energy for recrystallization of 3.7+/-0.2 eV, which is higher than what has been previously suggested. (C) 2003 Elsevier B.V. All rights reserved.