The chemical equilibrium of hydrogen and aqueous solutions of 1:1 bicarbonate and formate salts with a common cation has been investigated in an intensively stirred batch reactor : MHCO3(aq) + H-2(aq) <----> MOOCH(aq) + H2O(1) This was accomplished for the sodium (M = Na), potassium (M = K) and ammonium (M = NH4) systems by measuring the equilibrium composition. This reaction was allowed to proceed from both sides of the equilibrium in a suspension of Pd/C and Pd/gamma-Al2O3 catalyst particles, and was carried out at 20, 40 and 60 degrees C for hydrogen pressures ranging from 0.5 to 10 bar. The total salt concentration in the reaction mixture was varied up to the solubility limit. The experimental equilibrium data were interpreted by taking the nonideality of the aqueous electrolyte solution into account according to the multicomponent, extended Fitter model. Calculation of the activity coefficients was based on parameters of the single electrolyte subsystems. From the experimental results, the values of the standard Gibbs energy and enthalpy change of the reaction could be derived, namely -0.72 and -20.5 kJ mol(-1), respectively. Determination of the standard Gibbs energy and enthalpy of formation of the aqueous formate ion yielded values of, successively, -350.5 and -426.7 kJ mol(-1). The thermodynamic chemical equilibrium constant of the hydrogenation reaction was correlated by the equation K-1 = exp (2.22 x 10(3)(T/K)(-1)). In the range of process conditions studied, both the equilibrium conversion and the solubility of the electrolyte mixture were predicted within 5% error on the basis of the thermodynamic model used.