When the pH is specified, the criterion for spontaneous change and equilibrium is stated in terms of the transformed Gibbs energy G＇ that is defined by a Legendre transform introducing pH as an intensive variable. The change in the standard transformed Gibbs energy of reaction Delta(r)G＇degrees gives the apparent equilibrium constant K＇, and the standard transformed enthalpy Delta(r)H＇degrees gives the temperature coefficient of K＇. The corresponding standard transformed entropy of reaction Delta(r)S＇degrees is less often discussed but is often easier to interpret in terms of structure. The standard transformed entropies of formation of 44 biochemical reactants are calculated at 298.15 K, 1 bar, pH 7, and ionic strengths of 0, 0.10, and 0.25 M from values of the standard Gibbs energies of formation Delta(f)G degrees and standard enthalpies of formation Delta(f)H degrees of the species involved. These values are used to calculate Delta(r)S＇degrees for 19 biochemical reactions to illustrate the role that entropy plays in these reactions. Delta(f)S＇degrees values for biochemical reactants at a specified pH can also be calculated from standard molar entropies of the species involved in a reactant. The standard transformed formation properties of biochemical reactants can be estimated by adding up group contributions. The contributions of 15 groups to Delta(f)G＇degrees and Delta(f)H＇degrees at pH 7 and three ionic strengths are calculated and used to estimate Delta(f)G＇degrees and Delta(f)H＇degrees for seven reactants. This shows that standard transformed formation properties call be estimated for biochemical reactants for which there are no equilibrium or calorimetric data.