The metalorganic vapor-phase epitaxy of GaAs on Al2O3 substrates was carried out over a growth temperature range of 600-800°C. Several physical characterization techniques were used to obtain information on the initial stages of epitaxial growth and its influence on the physical properties of the epitaxial layer. The initial GaAs growth proceeds by island formation and coalescence, and the initial density of GaAs islands is a strong determinant of the epitaxial layer crystalline quality. An incommensurate growth model is presented to explain the dependence of the film properties on the growth parameters. The presence of defects in even very thick layers limits the applicability of these materials in device structures.