Mixed membranes composed of various fractions of gelatin and chondroitinsulfate C (chs) were made insoluble by formaldehyde. The swelling of these in aqueous media were experimented in the range of pH 2.9-7.0. The rate constant (k) of swelling (obtained from the equation of the first-order reaction), the rate of increase in membrane thickness (α_L) and that in membrane radius (α_r) in the initial step of swelling were obtained. With increasing weight fractions of chs (X_chs), k increased in the case of pH 5.6 and 4.2, but decreased when pH was 2.9. It was estimated from the analysis of these results that the insoluble complex formation between gelatin and chs occurs and depresses the swelling rate. The swelling of these membranes were anisotropic, and the values of α_L and α_r depended on X_chs as well as on pH in a similar behavior as that of k.