The vapor pressure of nickel carbonyl was determined with material obtained by laboratory preparation with CO and Ni powder and by purifying commercial nickel carbonyl. The observed vapor pressure data were used in the derivation of the following equations which express the vapor pressure, P, of liquid nickel carbonyl and the sublimation pressure, P/sub s/, of the solid, respectively, as a function of the absolute temperature, T: log P = 7.88431578/T and log P/sub s/ 10.1897-2173/T. The average heat of vaporization is calculated to be 7.22 plus or minus 0.01 kcal per mole and the molecular heat of sublimation of Ni(CO)/sub 4/ is 9.94 plus or minus 0.11 kcal. Extrapolation to 760 mm Hg gives a boiling point of 42.2 deg C. The melting point of purified nickel carbonyl occurred at -17.2 deg C. The sensitivity of the melting point to the effect of impurities was demonstrated with commercial nickel carbonyl which melted at-19.4 deg C before purification, at --18.3 deg C after distillations at 0 deg C, and at -17.2 deg C after sublimation at -25 deg C. The vapor pressure of liquid nickel carbonyl at 0 deg C is of little value in evaluation of the purity of …