Apparent equilibrium constants and calorimetric enthalpies of reaction have been measured for reaction a catalysed by branched-chain-amino-acid transaminase. The following biochemical reactions have been studied at the temperature 298.15 K and in the pH range (7.15 to 7.24): L-valine(aq) + 2-oxoglutarate(aq) = 2-oxoisovalerate(aq) + L-glutamate(aq); L-leucine(aq) + 2-oxoglutarate(aq) = 2-oxoisocaproate(aq) + L-glutamate(aq); and L-tert-leucine(aq) + 2-oxoglutarate(aq) = 3,3-dimethyl-2-oxobutanoate(aq) + L-glutamate(aq). The results have been used to calculate equilibrium constants and standard molar enthalpy Delta H-r(m)o, entropy Delta S-r(m)o, and Gibbs free energy Delta (r)G(m)(o) changes for reference reactions involving specific species. Apparent equilibrium constants and standard transformed Gibbs free energy changes for these reactions under physiological conditions have also been calculated. The use of these results fur optimization of product yields of the branched-chain amino acids is discussed.