The effect of the sample size on the retention time is studied both experimentally and theoretically. It has already been shown that the deviation of the retention time from its value at infinite dilution is a consequence of a nonlinearity in the basic transfer relations. The previous approach is extended to systems for which the equilibrium isotherm is no longer linear. The relative deviation is either positive or negative according to the curvature of the isotherm. A good agreement between predicted and experimental values is obtained.