The relative volatility or separation factor for deuterium enrichment in ammonia distillation was measured at pressures of 250, 375, 500, 600, and 760 mm Hg and at deuterium concentrations of 0.10, 0.24, 0.42, and 0.58 mole fraction deuterium. The measurements are summarized by the following equation: In (α) = (0.0395 ±0.0004) − (0.0128 ±0.0029) (x− 0.424) − (0.01246 ± 0.00065) (lnπ/760 mm Hg) where α = separation factor π = system pressure mm Hg x = mole fraction deuterium. It is interesting to note that a dependence on the composition was observed. This is not predicted by the normal method of calculating the separation factor from the vapor pressure ratio However, the magnitude of the separation factor and its dependence on pressure are in good agreement with the vapor pressure ratio predictions (α = 1.042 at 1 atm). This information is helpful in predicting costs of heavy water production by ammonia distillation. It has been stated by Barr and Drews (3) that ammonia distillation would be competitive with other developed methods only if the actual separation factor was at least 1.062 at low deuterium concentration. Since the separation factor observed was only 1.042 at atmospheric pressure, ammonia distillation is not an economic method for producing heavy water. Deutero-ammonia was synthesized by isotopic exchange between natural ammonia and heavy water. Equilibrium determinations were made using an Othmer still modified for low temperature operation. The ammonia samples were analyzed for deuterium content by converting them to water by passage over hot copper oxide, followed by a differential density determination using the falling drop method.