The protonation equilibria of tetrakis(4-sulfonatophenyl)porphyrin sodium salt, Na(4)(H(2)tpps), and its complex formation with the Cd(2+) ion at different ionic strengths [(0.1 to 1.0) mol.dm(-3) NaClO(4)] were studied over a wide pH range (2 to 12) using a combination of potentiometric and spectrophotometric methods at 25 degrees C. Least squares regression calculations are consistent with the formation of the Cd-Htpps(3-) and Cd-tpps(4-) species. The hydrolysis of cadmium ion was also investigated specrophotometrically at different ionic strengths of sodium perchlorate, and four hydrolytic species were specified in the experimental conditions. The dependence of protonation, hydrolysis, and the fort-nation constants of the porphyrin and the metal ion on ionic strength is described by a Debye-Huckel-type equation, and the results have been. compared with data previously reported and interpreted.