The multiple ionic forms of metabolites were evaluated at 37 degrees C for four reactions important in muscle contraction and recovery: 1) ATPase, 2) creatine kinase, 3) the Lohmann reaction, and 4) the Lohmann reaction reversed by coupling to glycogenolysis and glycolysis. Solution of the system of equations defining the multiple equilibria of the proton and cation complexes gives the concentration of each ionic form and a value for the proton stoichiometry for each reaction. The proton stoichiometric coefficients are unique for each reaction and are a function of pH because of differential binding of Mg2+ and K+ to adenine nucleotides, phosphocreatine, and Pi and because of different acidic dissociation constants for the metabolites. These results show the need to consider the binding of K+ in addition to the previously documented effects of Mg2+ in the cytoplasmic milieu. Commercially available software was used to show that related problems can be calculated readily on personal computers in applications similar to those described here.