A new method is presented to interpret data on diffusion-sorption processes obtained from through-diffusion experiments. Both sorption and desorption are modeled as kinetically controlled processes by incorporating nonlinear isotherms. The diffusion-sorption equation was solved numerically and the resulting algorithm was integrated into a nonlinear regression code to test various models. The through-diffusion technique was modified such as to obtain sorption and desorption data from a single experiment. Data are presented for the diffusion of cesium through a Eutric Cambisol soil pellet. Various sorption models were tested. The best fit to the experimental data was obtained assuming first-order kinetics with a Freundlich isotherm. Desorption shows a marked hysteresis.