Equilibrium water vapor pressures and kinetics of water sorption on potash fertilizers (KCl) were determined by a calorimetric method. Enthalpy changes for water sorption were estimated from the temperature dependence of equilibrium vapor pressures. Treatment of the fertilizers with anticaking agents had Ilo effect on the measured parameters. Anticaking agents apparently prevent cementitious crystals from growing between particles either by preventing contact of surface solution films or by modifying the crystal habit of the cementitious layer, instead of by slowing or diminishing the sorption of water.