The mass transfer behavior of solutes, including diffusion and membrane cutoff characteristics in capsules that were prepared by the interfacial reaction of sodium cellulose sulfate (NaCS) and poly(dimethyldiallyl-ammonium chloride) (PDMDAAC), was studied experimentally. By using the diffusion model developed, the diffusion coefficients of low-molecular-weight solutes, such as glucose, sucrose, ethanol, lactic acid, L-lysine, L-glutamic acid, L-tyrosine, L-tryptophan, and L-phenylalanine, in NaCS-PDMDAAC polyelectrolyte membrane were determined. The values of diffusion coefficients in the membrane were about 2-10% of those in pure water. The experimental results indicated that the encapsulation conditions, such as PDMDAAC concentration and PDMDAAC molecular weight, could affect the structure of NaCS-PDMDAAC membrane and further would lead to the change of diffusion coefficients of solutes in capsules. The molecular weight cutoff (MWCO) is less than 2 kDa, which indicates that the NaCS-PDMDAAC capsule membrane has good cutoff performance and can well obstruct the passing of the membrane by the high-molecular-weight solutes.