Direct impact of H-2 and N-2 diluents on the metal-organic-chemical-vapor-deposition gas-phase chemistry in M(CH3)(3)/NH3 (M = Al, Ga, In) systems is identified in the framework of Density Functional Theory in terms of cohesive energy differences. While both diluents destabilize model reaction species, i.e. adducts, transition states and chain complexes, the effect is particularly strong with respect to N-2 in the AI(CH3)(3)/NH3 system, and can be a factor to restrain the expansion of chain complexes that deplete the gas-phase from precursors. Theoretical results are supported by experimental evidences of higher growth rate and superior optical properties of A1N grown in N-2 vs. H-2 diluent. (c) 2006 Elsevier B.V. All rights reserved.