We have used gas-phase infrared spectroscopy to determine the equilibrium constant (K-p) for the formation of (CH3)(3)Ga:NH3 and (CH3)(3)In:NH3 adducts in the 80-230 degreesC range. In this temperature range, and at reactant concentrations typically used for metal organic chemical vapor deposition. the dominant chemical reaction is reversible adduct formation/dissociation. Reaction enthalpies and entropies are extracted from the temperature dependence of K-p, yielding DeltaH(Ga) = -16.3 +/- 0.5 kcal/mol, DeltaS(Ga) = -32.4 +/-1.2 eu, and DeltaH(In) = -15.0 +/- 0.6 kcal/mol, DeltaS(In) = -30.3 +/-1.4 eu. These results will aid current and future modeling efforts, as well as advance our general understanding, of the 2,group-III nitride deposition process.