UV-vis spectra of 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) have been studied in dilute (1.0 muM) aqueous solution at pH 7.0 as a function of added electrolytes, including phosphate buffer, methyl viologen dichloride (MV), and propyl viologen sulfonate (PVS). At concentrations less than or equal to 1.0 muM and pH greater than or equal to 7.0, TCPP is considered to be tetraanionic and unaggregated. It shows a strong B (Soret) band at 414.2 nm that can be decomposed into three Gaussians, and follows Beer's law with epsilon = 4.8 x 10(5) M-1 cm(-1) at pH 7.0 in 5.0 mM phosphate buffer. Deviations from Beer's law appear above 2 muM under the above conditions; linear regions for Beer's law plots depend on the pH, temperature, and ionic strength. Even around 1 muM, the Sorel absorption strength is a very sensitive function of electrolyte concentration; for example, epsilon = 3.6 x 10(5) M-1 cm(-1) at pH 7.0 in 62 mM phosphate buffer, and epsilon = 5.1 x 10(5) M-1 cm(-1) in aqueous solution of minimal ionic strength (0.1 mM NaOH, pH 10). In the titration of TCPP with MV, two types of experiments were performed, at low ionic strength (which necessarily increased in the later stages of the titration) or at high (effectively constant) onic strength. In 5.0 mM phosphate buffer at pH 7.0 two isosbestic points are observed, at 418.4 nm (in the range of 40-300 muM MV) and at 421.5 nm (15-90 mM MV), considered to represent the formation of 1: 1 and 1:2 complexes (porphyrin-viologen). By nonlinear regression analysis, the complexation constants are calculated to be K-1 = 3350 M-1 and K-2 = 68 M-1. Application of principal component analysis identifies the spectra of both complexes and gives K-1 = 2200 M-1 and K-2 = 100 M-1. The complexation constants, the spectra of the complexes, and the positions of the isosbestic points vary with the buffer concentration. At 62 mM phosphate buffer, K-1 = 600 M-1 and K-2 = 40 M-1 by nonlinear regression analysis (K-1 = 1200 M-1 and K-2 = 140 M-1 by principal component analysis). H-1 NMR spectra indicate that both TCPP and MV undergo concentration-dependent resonance shifts that can be correlated with the calculated complexation constants. Similar titration of TCPP with PVS also shows evidence for 1: 1 and 1:2 complexes, but with lower equilibrium constants compared to those from titration with MV. At high PVS concentrations (>25 mM) a blue-shifted Soret band attributed to TCPP dimer is observed. The same spectrum is observed under a variety of other conditions, including the presence of a cationic surfactant (4 muM CTAB), very high ionic strength (3 M NaCl), or a polycationic complexing agent. These effects illustrate the variety of influences that affect the porphyrin absorption spectrum in aqueous solution and offer cautions about the conditions of experimental control and data analysis that are necessary to extract meaningful titration data.