Using an atomic force microscope operated in contact and noncontact mode, we have studied the structures formed on the (100) cleavage surface of NaCl when exposed to water vapor. Above similar to 35% relative humidity (RH), a uniform layer of water is formed and the surface steps are observed to evolve slowly. At similar to 73% RH, the step structure becomes unstable and disappears abruptly because of dissolution (deliquescence) of the salt surface. Reversing the process by drying leads to the reappearance of new, more uniform monatomic steps. At humidity levels less than 30%, water adsorbs primarily at the step edges.