The paper gives results of measurements of the effects of water vapor and CO2 on the sintering rate of nascent CaO, as a function of partial pressure and temperature using CaO prepared by rapid decomposition of CaCO3 and CA(OH)2. Each gas strongly catalyzed the sintering process and their combined effects were additive. A model formulated by R. M. German and Z. A. Munir in 1976, which describes surface area reduction by a single mechanism, is empirically modified to account for catalytic effects of the gas phase. Although multiple sintering mechanisms are apparently operative in the presence of CO2 and/or H2O, the empirical model correlates isothermal surface area reduction as a function of time over the temperature range of 380 to 1150 C and partial pressures of 39 Pa to 15 kPa. Porosity reduction was also accelerated by the presence of H2O or CO2 in the sintering atmosphere. In an atmosphere of simulated flue gas, porosity reduction followed the Coble logarithmic law during sintering at 800, 900, and 1000 C with induction periods of 6, 4.5, and 2.7 sec, respectively, for onset of particle shrinkage. Coupling the sintering model with a sulfation model that accounts for the effects of surface area allows the prediction of SO2 capture efficiencies in boiler furnaces by Ca(OH)2 injection.