Films of alumina, magnesia, and silica were exposed to cesium atoms at room temperature. Cesium adsorption as a function of exposure time was studied with metastable impact electron spectroscopy (MIES) and photoelectron spectroscopy (UPS; He I). On silica, cesium atoms readily chemisorb in the initial stages of exposure. The bonding is apparently due to the interaction with active surface oxygen such as non-bridging oxygen atoms. We suggest that on alumina and magnesia, the chemisorption of cesium, in the form of an ionic state, takes place at surface defects, presumably edge sites such as steps, kinks, corners, etc. In all cases studied, prolonged exposure leads to additional Cs adsorption. Apparently, patches with metallic properties (but no uniform adlayer) develop on the surface; they disappear after the cesium supply is interrupted.