Legendre transforms are needed to introduce intensive variables into the description of thermodynamic systems. But when experimental thermodynamic data involve an inconveniently large number of intensive variables, inverse Legendre transforms are needed to reduce the number of independent intensive variables. This is illustrated by the elimination of pH in the calculation of standard Gibbs energies of formation of species from apparent equilibrium constants of enzyme-catalyzed reactions. Apparent equilibrium constants have been determined for about 500 enzyme-catalyzed reactions involving about 1000 different reactants at various pHs and ionic strengths. However, these apparent equilibrium constants cannot be compared directly with each other because of differences in pH and ionic strength. More than one pathway can be used to calculate standard Gibbs energies of formation of the species involved. This discussion of inverse Legendre transforms provides guidance on how to write computer programs to eliminate pH as an independent variable and obtain standard Gibbs energies of formation of the species involved. This is illustrated by calculation of standard Gibbs energies of formation of species of phosphoenolpyruvate, 2-phospho-D-glycerate, 3-phospho-D-glycerate, and 3-phospho-D-glyceroyl phosphate at zero ionic strength and 298.15 K. These calculations show that the reactions in a series such as glycolysis do not necessarily provide the best route for the calculation of standard transformed Gibbs energies of reaction, even though values of apparent equilibrium constants of these reactions have been reported.