The adsorption of water on several alkali halide surfaces was studied using scanning polarization force microscopy. Water adsorption leads to an overall increase of surface potential and ionic mobility. At a critical humidity that is characteristic of each salt (NaCl, KCI, KBr, and KI), important changes in the rate of increase of the surface potential and ionic mobility occurred. Topographical changes occurred as well, in the form of step motion. These were observed to occur at a fast rate above the critical point, while little step motion occurred at lower humidity. Dissolution of the crystal (deliquescence) is observed at high humidity. Below the critical humidity, contact potential images indicate that preferential solvation of cations occurs at steps producing a large enhancement of the step contrast in the images.