A fundamental reaction-transport model describing the metalorganic vapor-phase epitaxy (MOVPE) of GaN from trimethyl-gallium (TMG) and ammonia has been developed. This model has been tested against experimental data from research-scale and industrial-scale reactors. A simplified version of the model that includes only transport phenomena and a unity sticking coefficient of the limiting film precursor (TMG) to the surface of the growing film was found to accurately capture observed film deposition variations in an early variant of the Thomas Swan close-coupled-shower-head 3 x 2 " reactor. Modifications of the Thomas Swan reactor, in line with the findings suggested by this work, enabled state-of-the-art thickness uniformity to be achieved. The model has been used to develop performance diagrams for conceptual multi-aperture MOVPE reactors and for the Thomas Swan system. These performance diagrams identify regions of the parameter space of the reactor which correspond to minimal variations in film growth rate across large-area substrates. Published by Elsevier Science B.V.