Accurate evaluation of qq=qC has been a serious problem in the accuracy of previous combined diffusion models. The combined diffusion model developed by Neretnieks (1976) was based on determining the variable effective diffusion coefficient De, and then using the relationships, Des ¼ Ds þ Dpep=K; and Dep ¼ DsK þ Dpep. The two effective diffusion coefficients differ by the factor K. The paper then considered the methods for expressing K for a linear isotherm, a Tempkin isotherm and a Freundlich isotherm. Both the Tempkin and Freundlich isotherms had to be modified to yield acceptable diffusion coefficients. This model was applied to six experimental systems using activated carbon from previous research by Spahn et al. (1975). For two