Calorimetric and potentiometric titrations have been used to evaluate direct and indirect methods of analysis for the carboxyl content of aquatic humic substances. The effects of cation binding, base strength, and removal of humic reaction products on carboxyl content values have been examined. The results suggest that the most appropriate analytical method for determination of an operationally defined carboxyl content would utilize a weak base that does not contain a polyvalent cation in an indirect titration on a reaction mixture from which all humic reaction products have been removed prior to titration. Titration data have been analyzed in terms of a formation function (δ n OH ) and in terms of operationally defined ‘mass action quotients’ ( K ̄ ). The dependence of K ̄ on pH is more readily and realistically described if no artificial differentiation of carboxyl groups into two or more sub-groups is imposed upon the data.