A direct discontinuous titration method with NaOH was developed to characterize the acid-base properties of complex natural polyelectrolytes, such as humic substances (HS). The classical batch procedure, suggested originally for examinations of resin-based ion exchanger, was modified to be applicable to HS; the modified method is suitable especially for the measurements of the titration curves of solid humic acids (HA). The titration data provide a valuable information on the nature and distribution of acidic functional groups in the HA molecules. The contents of carboxylic and phenolic groups in the HS molecules were calculated from the raw titration data and compared with the results obtained by the barium hydroxide and calcium acetate exchange methods. A good agreement was found for the contents of carboxylic groups, but not for the contents of phenolic groups. An uncertainty of the determination of acidic functional groups was estimated from the series of duplicate analyses of real coal-, peat- and soil-derived solid humic acids; the relative uncertainty of the determination of carboxylic groups was 1.8%, whereas the relative uncertainty of the determination of phenolic groups was 9.1%. (c) 2007 Elsevier Ltd. All rights reserved.