This study examines chemical properties of cationic biosorbents; orange and lemon peel. ATR spectra show the functional groups of tested biosorbent samples are protonated and deprotonated carboxyl and hidroxyl groups. In the NaOH solution with pH value 13, acidic –COOH groups are neutralized while the ester groups of carbohydrates are hydrolised. Because of the latter, this neutralization method of detection of carboxyl groups, otherwise widely recomended in literature, has not been accurate. Since the alkaline hydrolysis of sorbent produces extra deprotonated carboxyl groups (–COO-), the amount of cationic dye methylene blue sorbed by biosorbents is greater after the biosorbents have been processed with the NaOH solution. Potentiometric and conductometric titration of biosorbent suspensions in distilled water, with varying masses of biosorbent, shows a reduction in the degree of dissociation of –COOH groups with the growth of the biosorbent mass, so the portion of H+ ions that can be exchanged with cations in the solution diminishes. In the HCl solution with pH value 1 the alkaline –COO- groups are neutralized and this neutralization method of detection of –COO- groups turned out to be correct. The isotherms of methylene blue sorption by the biosorbent samples are S-shaped, and this can be ascribed to the sorption of methylene blue by protonated and deprotonated carboxyl groups in solutions with dye concentrations lesser than 800 mg/dm³, and by –OH groups in solutions with dye concentrations greater than that. Sorption of methylene blue can be used as a method of determination of total quantites of –COO- and –COOH groups.