Water sorption isotherms of NPK 10-20-20/4 muriate of potash (MOP) fertilizer were determined using a gravimetric method for temperatures ranging from 15 to 35 degrees C. It was found that the sorption behavior of the Fertilizer assimilated to a type-Ill isotherm characteristic of crystalline solids. The sorption data Could be represented by a single isotherm in the temperature range Studied. The equilibrium data obtained were fitted to two-parameter linearized and nonlinear three- and four-parameter models. Among the models studied, the nonlinear three-parameter Guggenheim-Anderson-De Boer (GAB) model was selected. This equation allowed the calculation of a monolayer water content of 0.852 g of water/100 g of dry solids, and monolayer and multilayer water sorption enthalpy values of H-N = 43.3 kJ/gmol and H-M = 44.2 kJ/gmol, respectively.