The combination of the gaseous species of a conventional MOCVD system with a UHV chamber is described. Thermal decomposition of arsine (AsH3) and trimethylgallium (TMG) has been analyzed by mass spectrometry. An activation energy of 35 kcal/mole is found to be associated with arsine decomposition. The possibility of GaAs growth in high vacuum from pure TMG and elemental arsenïc (As4) source is demonstrated and growth kinetics in both (TMG, As4) and (TMG, AsH3) systems is investigated. In particular, the growth mechanisms involved let the possibility for localized epitaxy