GaAs surface composition changes occurring during Al film growth using the CBE method with laser assistance were investigated in situ by means of laser reflectivity. The results were compared with data on precise chemical analyses and X-ray microanalyses carried out after film deposition. It was found that the peculiarity of film formation depended upon the laser power. Physicochemical interactions of the Ga atoms from the GaAs surface, with atoms and molecules from the surrounding media, are determinative reactions at a laser power of 2W. At a power of 0.02W, the laser reflectivity changes were mainly due to reactions with Al. The appearance of “free” Ga and As in the region outside of the laser spot indicated the destruction of GaAs islands weakly connected with the GaAs surface.