The authors report a study comparing the surface reactivities of the alkaline earth oxides MgO and CaO with respect to water vapor under ultrahigh and high vacuum conditions. Using x-ray photoelectron spectroscopy and a series of in vacuo exposures spanning similar to 10(-10) Torr vacuum, to 10(-6) Torr flowing oxygen, the extent of surface hydroxylation was investigated by monitoring the O 1s photoelectron line. After the most aggressive exposures, the MgO surface reacted to form a maximum hydroxyl coverage of similar to 1.3 ML (monolayer), while the CaO surface reached a maximum coverage of similar to 3.0 ML. Both surface hydroxides could be removed by in vacuo thermal anneals; exposure to 250 and 500 degrees C were required for MgO and CaO, respectively. These results are framed in terms of the suitability of these oxides in complex oxide-compound semiconductor heteroepitaxial multifunctional devices and with respect to understanding the development of surface morphology in epitaxial 111-oriented CaO and MgO thin films.