The conception of oxidation-reduction potentials as a measure of differences of free energy in hydrogen transport systems is derivable only from equilibrium states. The equations used to describe the effects of varying concentration ratios are based upon the law of mass action and refer only to ideal solutions. When methods of analysis derived from oxidation-reduction potentials are applied to the living plant cell it has to be accepted that in the equations which may be used, referring to equilibrium states, the time factor is absent and also any molecular orientation factor. The processes in the cell have to be expressed not in terms of equilibria but rather in terms of steady states. Unlike an equilibrium state, which is unaffected by whatever mechanism we imagine to be operative, the steady state depends directly upon the reaction mechanism which has to involve both a time factor and molecular orientation factors. The knowledge of the characteristic potentials of the substances responsible for hydrogen transport can, however, tell us exactly how far a reaction between components can proceed when equilibrium between them is reached. Thus the measurement of differences in oxidation-reduction potentials supplies a fundamental basis for relating many of the dynamic processes in the plant cell.