The commercially applied procedure for GaN deposition using Ga(CH3)(3)/NH3 was investigated in a wide temperature region between 300 and 1500 K. Molecular beam sampling using quadrupole mass spectrometry has been used to show that gallium-nitrogen compounds, like the Lewis-acid-base adduct (CH3)(3)GaNH3 and dimeric clusters like [(CH3)(4)Ga-2(NH2)(2)], [(CH3)(3)Ga-2(NH2)(2)] and [(CH3)(3)Ga-2(NH)] appear in the boundary layer of a sapphire substrate in the temperature range between 300 and 1000 K, whereas above 1000 K the only species detected are Ga(CH3)(2), Ga(CH3) and mostly Ga atoms. Therefore we conclude that mainly Ga atoms carry the flux of metal onto the substrate where epitaxial growth takes place (> 1200 K).