Cation diffusion coefficients in clayey materials partly appear to be greater than diffusion coefficients of water tracers. The measured values vary between experiments performed at different salinities or different tracer concentrations. This effect is especially pronounced for cations that sorb strongly on the clay surfaces, such as Cs. The observations illustrate the difficulties in applying Fick's law to cation diffusion in clays and demonstrate the need to find a consistent description of cation diffusion in clays that can be used to predict experiments performed at different conditions. In order to consistently describe Cs diffusion in Opalinus Clay, a multi-site surface diffusion model was implemented in the continuum-scale reactive transport code Flotran. The model combines pore and surface diffusion in one single diffusion coefficient, which accounts for the diffusion of sorbed cations along the clay surfaces. The contribution from surface diffusion to the diffusion coefficient is directly coupled to the sorption behavior via the derivative of the sorption isotherm. The model parameters include the surface mobilities, which are specific for each cation and sorption site. To derive surface mobilities for Cs, in-diffusion experiments were conducted at eight different stable Cs background concentrations. A set of surface mobilities for Cs on three sorption sites in Opalinus Clay was estimated by fitting the surface diffusion model simultaneously to these experimental data. Moreover, the sensitivity of the model to sorption parameters and surface mobilities was evaluated. The surface diffusion model with the estimated surface mobilities was then successfully tested against independent experimental data for Cs in Opalinus Clay, illustrating the model's predictive capabilities.