An approach to the analysis of isotherms of cooperative binding is developed that allows to calculate approximately affinity profiles, i t . dependencies of binding constants on binding densities. The comparison of the affinity profiles of the interactions of Ca2+ and S?-+ ions with sodium pectate as well as sodium alginates with the different content of the blocks of a-Lguluronic acid residues (GG-blocks) and blocks of mixed composition showed that (1) pectate has higher affinity to both ions as compared with alginates; (2) the affinity of pectate to CaZ+ ions is comparable to its affinity to S?+ ions but in the case of Ca2+ ions the maximum of the affinity profile falls on the substantially lesser value of the binding density in com-parison to S?-+ ions that seems to be unfavorable from the standpoint of the use of this polyuronide in preventive or medicinal nutrition; (3) the affinity of both alginates to S?-+ ions at relatively low binding densities exceeds their affinity to Ca2+ ions; (4) the affinity of alginates to sZ+ ions increases with the increase in the content of GG-blocks, whereas the affinity to Ca2+ ions practically does not depend on the composition of the alginate; (5) the maximum binding density of S?-+ ions to the alginates approximately corresponds to the content of GG-blocks. These results corroborate the practice of the use of alginates rich in residues of a-L-guluronic acid for the removal of radioactive strontium from the digestive tract. The binding constants of Pbz+ ions to pectate over a wide range of binding densities are more than an order of magnitude greater than those of alkali-earth metal ions.