Theoretical calculations were made to determine nitriding potentials at reduced pressure as a function of ammonia content for ammonia-hydrogen mixtures.  The α-γ’ and γ’-ε phase boundaries were calculated at 125 torr, in terms of the furnace input nitriding potentials.  The accuracy of the calculations was verified by experimentation.  An ammonia dissociation burette was used to determine the degree of dissociation.  The addition of hydrogen gas to ammonia requires a correction for determining the degree of dissociation from the burette readings.  An equation was developed that accounts for hydrogen additions in calculating nitriding potentials.  Light microscopy was used to evaluate the structures of nitrided 4140 steel.  In addition, tests were made to determine corrections that should be made to compensate for the fact that in pumping-down of an ammonia-hydrogen mixture, the input ratio of ammonia to hydrogen is reduced.  These data were used for calculating, at any reduced pressure, the low torr range critical values of the nitriding potential in terms of the input mixtures of ammonia and hydrogen.