Mixed membranes were prepared from gelatin and chondroitin sulfate C (chs) whose fractions (X_chs) were varied from 0 to 0.317. They were cross-linked by formaldehyde. The membrane potential across these membranes was measured in the systems of KCl, NaCl, LiCl, CaCl₂, and MgCl₂ solutions. The experimental temperature was 25°. The theoretical values of membrane potential were obtained by summing up a diffusion potential and Donnan potentials at membrane-solution interfaces. The experimental values were in agreement with the theoretical ones in a high concentration range of the solution. From these results, the membrane charge densities (θ) were determined. θ values obtained were negative and decreased with X_chs. They depended on the kind of electrolyte solutions, and this was considered to result from the difference in the bonding rate of cation to chs. The bonding order was [chemical formula]. pH dependence of θ was also examined in acidic region in which gelatin and chs from complexes. It was found that the pH making θ equal to 0 decreased with X_chs.