We have studied GaN films grown on a-plane sapphire substrate by both hydride vapour phase epitaxy (HVPE) and metalorganic vapour phase epitaxy (MOVPE) using X-ray diffraction and transmission electron microscopy. The in-plane orientation relationships between the epitaxial films and the substrate are determined to be [1 1 2¯ 0]GaN?[0 0 0 1] sapphire and [1 1¯ 0 0]GaN?[1 1¯ 0 0] sapphire in the HVPE growth, while [1 1¯ 0 0] GaN?[0 0 0 1]sapphire and [1 1 2¯ 0] GaN?[1 1¯ 0 0]sapphire are found in the MOVPE growth. In a few films of both types a simultaneous presence of two domains, representing the two in-plane relationships, is observed although the preferable one in each type of films is strongly dominating. We propose that the two in-plane orientations of GaN are generally possible on the a-sapphire substrate and are related to the atom termination of the sapphire surface determined by the substrate pre-treatment used in the different growth methods. © 2003 Elsevier B.V. All rights reserved.