The effect of water vapor on the initial sintering kinetics of calcium oxide was studied at 920” to 1123°C. In the range of PHI() from 8 × 10W4to 487 mm Hg, water vapor increased the rate of sintering. The initial sintering kinetics followed the model for volume diffusion with calculated diffusion coefficients dependent onPH2O, which varied from (P,H2O)1/2at pressures between 6.1 and 50 mm Hg to (PIH2O)1/2at pressures between 244 and 487 mm Hg. The activation enthalpy varied from 123 ± 7 kcal/mole when PH2O= 0.04 mm Hg to 103 ± 3.5 kcal/mole at PH2O243.9