A dioctahedral bentonite was treated hydrothermally with KCl solutions (0.025-1 M) at different temperatures (60-200 degrees C) and times (1-360 days) to evaluate its use as a barrier for radioactive waste disposal. Equations relating adsorbed K on smectite with initial and equilibrium K concentrations in solution, temperature and time have been obtained. These equations are useful for modelling the influence and behaviour of these variables and contribute to the performance assessment calculations for radioactive waste repositories. Likewise, the thermo-dynamic parameters of the K adsorption process have been calculated. Other relationships, including silica in solution and pH, are also shown.