A procedure for pH-metric determination of the limiting adsorption of OH groups (A OH) by FeIII, ZrIV, CrIII, and InIII oxyhydroxide hydrogels from 0.1 and 1.0 M solutions of NaCl is described. Data on the molecular area occupied by a single OH group on the hydrogel surface (S OH) and the S spec values, which were calculated from A OH and S OH, are presented. The S spec value does not depend on the pH of hydrogel precipitation; the true S spec value can be determined only from sorption of the OH groups at the actual point of zero charge of the hydrogel. The A OH values for hydrogels were found to change only slightly during aging of hydrogels in electrolyte solutions.