In this work we investigate the initial stages of growth of heteroepitaxial films of InAs on semi-insulating InP substrates using MOCVD. Very thin films grown at different temperatures, Tn, were studied using TEM and SEM. The 3.2% lattice mismatch between InAs and InP causes strains that are partially accomodated by the generation of misfit dislocations at the interface. Our studies indicate that the deposited InAs nucleates in the form of islands which then grow and coalesce to form the film. Elemental mapping studies indicate that the area of substrate covered by the film is a function of the growth temperature. The thickness of the film and the electron mobilities also vary with T°. The films with best mobilities were obtained for Tn = 480°C.