Al films were grown by chemical vapor deposition at 400, 550, and 700°C on GaAs(100) substrates using the molecular precursor dimethylethylamine alane. The film morphology and composition were studied in situ by reflection high-energy electron diffraction and Auger electron spectroscopy, and ex situ by atomic force microscopy and scanning electron microscopy. Chlorine (at 400°C) and C and N (at 550 and 700°C) at or below the percent level were found to be the major contaminants of the deposited films. Systematic studies for deposition at 400°C established that the film microstructure evolves via the growth and coalescence of three-dimensional faceted islands with (100)Al∥(100)GaAs or (110)Al∥(100)GaAs preferential orientation. Coalescence of such crystallites was observed only for equivalent coverages of Al above 150 nm. Comparison with the microstructure of Al films obtained by evaporation suggests that in the temperature range examined the evolution of film morphology during chemical vapor deposition from dimethylethylamine alane was mainly determined by surface diffusion of isolated adsorbed Al atoms.