Complexation of poly(vinylpyrrolidone) and gelatin with certain metal chlorides (HgCl2, CdCl2, CoCl2, and ZnCl2) have been investigated by viscosimetric and spectrophotometric techniques in aqueous solutions. While the change in intrinsic viscosity, [eta], of poly(vinylpyrrolidone) has shown a discontinuity with a concentration of metal chlorides, gelatin showed a steady decrease : with increasing metal chloride concentration. The decreasing order of effectiveness of cations in complex formation is Hg2+ > Cd2+ > Co2+ > Zn2+ for poly(vinylpyrrolidone) and Zn2+ > Co2+ > Cd2+ > Hg2+ for gelatin solutions. It has been suggested that the poly (vinylpyrrolidone)/metal cation interaction is a charge-controlled reaction, and gelatin/metal cation is a covalent coordination in character. A similar metal cation effect has been observed for poly (vinylpyrrolidone) by UV-VIS spectrophotometry.