The protonation equilibria of tetrakis(4-sulfonatophenyl)porphyrin sodium salt, Na-4[H(2)tpps], and its complex formation with Zn2+ ion in aqueous solutions were studied over a wide range of pH values, (2 to 12), using a combination of spectrophotometric and potentiometric methods at constant temperature, 25 degrees C, and constant ionic strength (0.1 mol.dm(-3) sodium perchlorate). Least squares regression calculations are consistent with the formation of Zn(H2O)(2)L, Zn(H2O)(OH)L, and Zn(OH)(2)L, species, where L represents tpps(6-). The hydrolysis of zinc ion was also investigated spectrophotometrically in aqueous solution and four hydrolytic species were specified in the mentioned experimental conditions. Finally, the species mole fraction of the hydrolyzed and the complex species were plotted at different pH values and discussed.