The thermodynamics of the nitrogenase reactions are discussed in terms of chemical equations and biochemical equations. Chemical equations balance all elements and electric charge. Biochemical equations represent changes at specified pH and specified free concentrations of metal ions that are bound by reactants, but they do not balance hydrogen or metal ions that have specified free concentrations. At a specified pH, it takes three separate biochemical equations to represent the changes catalyzed by nitrogenase. [formula; see text] The first two equations are required because the nitrogenase and hydrogenase activities of the enzyme have not been separated. The hydrolysis of ATP is necessary, but it is not coupled stoichiometrically to the first two equations. The function of the hydrolysis of ATP by nitrogenase may be to provide the 10 H+ required per mol of N2 consumed. However, reactions cannot generally be coupled stoichiometrically through H+ because H+ is potentially available by dissociation of protein, buffer, and H2O. The standard Gibbs energies of formation of the reactant species are calculated for 25 degrees C, 1 bar, and ionic strengths of 0 and 0.25 M. The standard transformed Gibbs energies of formation of the reactants are calculated at 25 degrees C, 1 bar, pH 7, and ionic strengths of 0 and 0.25 M.