Surface chemical reactions in the ALD of GaN from TEG and NH3 plasma were experimentally examined using mass spectrometry. The reactions were monitored using m/z ratios of 29 and 30, corresponding to C2H5 and C2H6, respectively. We suggest that the TEG pulse resulted in a bond cleavage between the ethyl group and the surface, and the ethyl groups were removed either as intact ligands or through reductive elimination as ethane, thereby rendering diethylgallium on the surface. The second half of the reaction resulted in the formation of GaN monolayer formed by ligand exchange or hydrogenation reactions, releasing more ethyl ligands and rendering an NH2 terminated surface.