We have discussed the gas-phase parasitic reactions in MR3/H-2/NH3 (M=Al, Ga, In; R=CH3, C2H5) systems following the elimination of methane or ethane in terms of the regional density functional theory by carrying out ab initio quantum chemical calculations. It is clearly shown that Al source gases enhance reactivity for both of R = CH3 and C2H5. Furthermore, the difference in reactivity between and M(C2H5)(3)/H-2/NH3 systems is discussed. We found that the substitution of ethyl group, for methyl in metal source strengthens the M-N coordination interaction, and the tendency to reduce activation energies of the elimination reaction is observed. However, the steric repulsion between ethyl groups and ammonia, which hardly takes place for R = CH3, raises activation energies in the presence of excess ammonia. (C) 2002 Elsevier Science B.V. All rights reserved.