All of the equilibrium data on the ammonia synthesis reaction, which cover an extreme temperature range of from 325\ifmmode^\circ\else\textdegree\fi{} to 952\ifmmode^\circ\else\textdegree\fi{}C and an extreme pressure range (at some temperatures) of from 10 to 1000 atmospheres, are correlated with the compressibility and heat capacity data on the pure reacting gases by means of a rational and relatively simple equation for the mass action function ${K}_{p}$. In this equation the effect of temperature and of pressure on ${K}_{p}$ are separated, the relation for the temperature effect being exact, but certain approximations being introduced for the sake of simplicity into the relation for the pressure effect.All of the data on the ammonia synthesis equilibrium are represented within the experimental accuracy by the use of only two adjustable constants.