Sodium hydride dissolves in and reacts with molten NaOH to give an equilibrium mixture of NaH, NaOH, Na/sub 2/O, Na, and H. In the case where there is a gaseous phase (hydrogen) and only one condensed phase, the system is defined by the temperature, pressure, and one composition variable. The equilibriu, H/ sub 2/ pressure, which is a measure of the H/sub 2/ activity within the melt, was determined as a function of the composition of the condensed phase(s) at 600, 700, and 500 deg for equilibrium mixtures with original compositions of 2.5 to 97.5, 5.0 to 95.0, l0.0 to 90.0, and 20.0 to 50.0 mole% NaH-NaOH. The equilibrium H/sub 2/ pressure-composition isotherms obtained by removing measured increments of H/sub 2/ were reproduced by reabsorbing H/sub 2/. Results for the 5.0 mole % NaH mixture were duplicated by starting with an equivalent quantity of either Na in NaOH or Na/sub 2/O in NaOH, and reacting with measured increments of H/sub 2/. The system is discussed in relation to the interdependent reactions involved, the phase rule, the thermodynamics of certain reactions, and experimental techniques employed. (auth)