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) over bar 0](GaN)\\[0 0 0 1](sapphire) and [1 (1) over bar 0 0](GaN)\\[1 (1) over bar 0 0](sapphire) in the HVPE growth, while [1 (1) over bar0 0](GaN) \\[0 0 0 1](sapphire) and [1 1 (2) over bar 0](GaN)\\[1 (1) over bar 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. (C) 2003 Elsevier B.V. All rights reserved.