The acid-base equilibria of rupatadine fumarate were investigated in the absence and the presence of differently charged anionic (sodium dodecyl sulfate), cationic (cetyltrimethylammonium bromide), and nonionic (4-octylphenol polyethoxylate) surfactants. The pK(a) values of rupatadine and fumaric acid were potentiometrically determined at 25 degrees C and at a constant ionic strength (0.1 M NaCl). The obtained potentiometric data were evaluated with use of the computer program Hyperquad. Ionization in the surfactant-free media was defined, and the effects of surfactants on protolytic equilibria of rupatadine were estimated, based on a shift of the apparent ionization constants determined in micellar solutions against the pK(a) values in water. The anionic SDS micelles caused an increase in the pK(a) values of all the rupatadine ionization centers (Delta pK(a) up to +1.44), while the shift of protolytic equilibria in different directions was observed in the case of the cationic CTAB (Delta pK(a) from -1.99 to +0.14) and the nonionic TX -100 (Delta pK(a) from -0.72 to +0.38) micelles. Distribution diagrams of the equilibrium forms as a function of pH indicate that the change in distribution is most strongly expressed in the pH range 4-8 which includes biopharmaceutically important pH values.