The presence of ammonia in a metalorganic vapor phase epitaxy (MOVPE) system configured for the growth of GaN films is necessary and sufficient to remove adsorbed O- and C-containing species from the (0001) surface of this compound without measurable decomposition of this surface. Chemical and microstructural evidence in support of this statement has been obtained from investigations concerned with the extension of a previously developed in situ process route for cleaning heated GaN(0001) surfaces in a molecular beam epitaxy environment using flowing ammonia to the higher pressure regime of MOVPE. Thin films of GaN were deposited under 20 Torr total pressure on GaN templates, previously exposed to the laboratory ambient, and heated in either a nitrogen/hydrogen or an ammonia/hydrogen mixture to the deposition temperature of 1020 °C. Secondary ion mass spectroscopy of these samples revealed significant concentrations of carbon and oxygen at the GaN/GaN interface in the former and the absence of these contaminants above the detection limits of the instrument in the latter. The surfaces of the templates heated in the nitrogen/hydrogen atmosphere also decomposed sufficiently to form a very thin liquid Ga layer that reacted with ammonia to form a GaN-containing film either at the outset of film growth or on cooling in an ammonia/nitrogen atmosphere. Atomic force microscopy (AFM) showed a smoother surface for the GaN films deposited on templates heated and cleaned in the ammonia/hydrogen mixture relative to films deposited on templates heated in the nitrogen/hydrogen mixture. The latter surface contained both a higher density of step terminations, indicative of a higher density of threading dislocations having screw and mixed character, and pits.