The difference between the hydraulic permeability K under a pressure gradient and the diffusive permeability P under a concentration gradient can be explained by the incipient viscous flow at high degree of swelling. This flow is opposed by the friction resistance of the macromolecules of the highly swollen membrane. It comes to an end at a critical swelling Hc when the number of permeant molecules is not more sufficient for a complete solvation of the macromolecules of the membrane. Below this swelling, K equals PV1/RT, where V1 is the molar volume of the permeant, and above it the difference K − PV1/RT is proportional to H/(1 − H) − Hc/(1 − Hc). The proportionality factor depends on the friction coefficient of the macromolecular segments and on the average lateral chain clustering. The data on poly(glycerol methacrylate) suggest that on the average the aggregates contain two chains.