The properties of epitaxially grown Al on InP(100) are investigated by in situ classical surface techniques and by ex situ TEM and x-ray diffraction. Particular emphasis is put on TEM which clearly shows the growth of Al monocrystalline islands confirming the RHEED and UPS results. In addition, TEM reveals microtopographical properties as well as more detailed aspects of the overlayer. On the basis of these observations, three stages are identified in the growth mechanism: up to a monolayer, a condensation phase with chemical reaction at the interface, then an island growth by diffusion relieved by coalescence leading to, finally, a uniform monocrystalline Al film. Although at the early stages of island growth two crystallographic orientations of Al are found, the final film is single crystal.