Reaction of water vapor at 25°CC with CaO powder of either high or low surface area yields Ca(OH)2 with broad XRD peaks. Relative rates of reaction of these hydroxides with CO2 at 250°CC depend mainly on the sample surface areas per unit weight. SEM observations show that the exterior shapes of porous 1 to 30 μm particles of CaO formed by decomposition in vacuum of CaCO3(sr‐CaO) or Ca(OH)2 (h‐CaO) are only slightly changed when they are converted to Ca(OH)2, except for surface roughening, which increases with time of exposure to water vapor. The N2 adsorption‐desorption isotherms of h‐CaO and sr‐CaO, and of their hydration products, are used to calculate the average particle expansion, the probable pore shapes, and the changes in pore‐size distributions that accompany the reaction. These data show that the reaction causes expansion perpendicular to ulterior surfaces of the porous powder particles. A possible mechanism is suggested. Both h‐CaO and sr‐CaO dissolve in liquid water to yield Ca(OH)2 by subsequent precipitation.