The thermodynamics of ideal and non-ideal solutions is applied to nickel hydroxide, the electrochemically active material in the positive electrode of a nickel battery, to provide theoretical insight into the reversible potential as a function of the state-of-discharge. The models were fit to experimental discharge curves to obtain the activity-coefficient parameters and the standard potential over a temperature range from 5 to 55 degrees C. The two-parameter activity coefficient models perform significantly better than the Nernst equation and one-parameter Margules at all temperatures, thus suggesting that the insertion of the protons into the lattice occurs in an ordered, non-random fashion. The standard potential decreases linearly with temperature by 0.84 mV/K. The thermodynamic expressions and parameters given here enable one to predict the reversible potential of nickel hydroxide as a function of temperature and state-of-discharge.