Comparison studies of the decomposition in the gas phase of pure phosphine and pure ammonia by cobalt-60 gamma rays show that both compounds are broken down to their elemental constituents. Results of mass balance experiments with pure phosphine indicate that the overall radiolysis reaction is ${\rm PH}_{3}\rightarrow {\rm P}_{\text{red}}+{\textstyle\frac{3}{2}}\ {\rm H}_{3}$. G for hydrogen production is 11.3 (molecules formed per 100 eV absorbed energy), over the dose range $0.4\times 20^{20}\ {\rm eV}$ to $5\times 10^{20}\ {\rm eV}$, beyond which the G value decreases. From 50 Torr to 1 atm the product yield is proportional to phosphine pressure, extrapolating to zero yield at zero pressure. Yields of nitrogen and hydrogen from the radiolysis of gaseous ammonia are proportional to dose from $1\times 20^{20}\ {\rm eV}$ to $5\times 10^{20}\ {\rm eV}$, with G values of 1.5 and 4.5, respectively. The total product yields extrapolate to zero yield at zero dose. The G values are not dependent on ammonia pressure from 100 Torr to 1 atm, and the product yields extrapolate to zero at zero pressure. Measurement of nitrogen by two methods gives very close agreement and indicates that the formation of hydrazine is insignificant. The absorbed dose rates were determined by measuring the hydrogen yield from irradiated ethylene, assuming a G value of 1.2 and electron stopping powers (relative to ethylene) of 1.041 for ${\rm PH}_{3}$ and 0.661 for NH₃.