The reactions of trimethylindium (TMIn) with H 2O and H 2S are relevant to the chemical vapor deposition of indium oxide and indium sulfide thin films. The mechanisms and energetics of these reactions in the gas phase have been investigated by density functional theory and ab initio calculations using the CCSD(T)/[6-31G(d,p)+Lanl2dz]//B3LYP/[6-31G(d,p)+Lanl2dz] and CCSD(T)/[6-31G(d,p) + Lanl2dz] //MP2/[6-31G(d,p)+Lanl2dz] methods. The results of both methods are in good agreement for the optimized geometries and relative energies. When TMIn reacts with H 2O and H 2S, initial molecular complexes [(CH 3) 3In1OH 2 (R1)] and [(CH 3) 3In:SH 2 (R2)] are formed with 12.6 and 3.9 kcal/mol binding energies. Elimination of a CH 4 molecule from each complex occurs with a similar energy barrier at TS1 (19.9 kcal/mol) and at TS3 (22.1 kcal/mol), respectively, giving stable intermediates (CH 3) 2InOH and (CH 3) 2InSH. The elimination of the second CH 4 molecule from these intermediate products, however, has to overcome very high and much different barriers of 66.1 and 53.2 kcal/mol, respectively. In the case of DMIn with H 2O and H 2S reactions, formation of both InO and InS is exothermic by 3.1 and 30.8 kcal/mol respectively. On the basis of the predicted heats of formation of R1 and R2 at 0 K and -20.1 and 43.6 kcal/mol, the heats of formation of (CH 3) 2InOH, (CH 3) 2InSH, CH 3InO, CH 3InS, InO, and InS are estimated to be -20.6, 31.8, and 29.0 and 48.4, 35.5, and 58.5 kcal/mol, respectively. The values for InO and InS are in good agreement with available experimental data. A similar study on the reactions of (CH 3) 2In with H 2O and H 2S has been carried out; in these reactions CH 3InOH and CH 3InSH were found to be the key intermediate products.