A method has been developed to predict sorption reactions of radionuclides on granitoid rock. This method is based on simultaneous ion exchange reactions for both radionuclide and competing cations in solution. Several batch sorption experiments using crushed and sieved rocks showed the Rothmund and Kornfeld type of equation, which determined exchange constants of cesium, cobalt, manganese, strontium, and competing major cations in natural water. Sorption data in three synthetic solutions of 1 N calcium chloride, groundwater, and seawater indicated that simultaneous ion exchange reactions predict mechanisms for all cations in solution. A simple method is proposed to predict a sorption isotherm of radionuclide in the presence of competing cations, such as potassium, sodium, and manganese.