Three kinds of poly(methylene-co-guanidine) (PMCG)-sodium alginate/cellulose sulfate capsules were prepared by using PMCG concentrations of mass percent volume of 0.2%, 0.5%, and 1.5%, respectively. The diffusivities of nine solutes with low molecular weight in the above capsules were measured. The diffusion coefficients of the solutes through the capsular membrane (D-1) and the combined diffusion coefficients between the capsular membrane and the intracapsular solution (D-m) were determined by established mathematical models. The calculated results were in good agreement with the experimental data, indicating that the mathematical model could well describe the diffusion characteristics in this capsule. D-1 is related to the PMCG concentration in encapsulation, and D-m has little relationship with the PMCG concentation. D-1 increased with the increase in PMCG concentration from mass percent volume of 0.2% to 1.5%, whereas D-m varied over a small range with the increase in PMCG concentration. The results of D-1 much less than D-m indicated that the resistance in mass transfer focused on the capsular membrane and the diffusion through the capsular membrane is the controlling step in mass transfer.