Based on the absolute-rate theory a consistent interpretation was provided for the dependence of surface-diffusion coefficient, D-s, on temperature and the amount adsorbed in various liquid-phase adsorption systems, such as the Langmuir-, Freundlich- and Jossens-type adsorption. It was demonstrated that a restricted molecular diffusion model for surface diffusion was useful for the analysis of the characteristic features of D-s. A formulation of D-s was derived based on the model and was applied to the analysis of surface-diffusion phenomena in various adsorption systems. The temperature dependence could be interpreted by assuming surface diffusion as an activated process. By taking into account the change in both the logarithmic slope of an adsorption isotherm and an adsorption potential, the concentration dependence of D-s could be interpreted irrespective of the type of the adsorption isotherms.