Porphyrins and viologens are often utilized in photocatalyzed electron transfer chemistry. Some of the problems faced by researchers in these applied studies include the lack of information correlating the porphyrin absorbance spectral changes with porphyrin dimerization, complexation, and the effect of noncomplexing cations. In addition, the efficiency of the system is greatly reduced by the formation of porphyrin:viologen association complexes. Knowledge of the values of these constants may allow selection of appropriate porphyrin:viologen concentration ratios which would enable optimization of these systems when used in such applications as artificial photosynthesis. This study reports the effect of selected salts and solvents on the porphyrin spectrum and the spectrum of the H₂TCPP⁴⁻ dimer. Moreover, we define experimental conditions appropriate to the study of porphyrin:viologen association and report association constants calculated at both constant ionic strength and constant buffer concentration. The spectrophotometric titration of tetrakis(pcarboxyphenyl) porphine (H₂TCPP⁴⁻) with either methyl viologen (MV²⁺) or propylviologen sulfonate (PVS⁰) results in the formation of H₂TCPP⁴⁻ :viologen complexes with stoichiometries of both 1:1 and 1:2. In addition, at high viologen concentration, PVS⁰ induces H₂TCPP⁴⁻ dimerization. Association constants for the titration of H₂TCPP⁴⁻ with MV²⁺ were calculated using data obtained at constant ionic strength (I = 0.15, maintained with KH₂P0₄/NaKHP0₄) and at constant buffer concentration ([KH₂P0₄/NaKHP0₄] = 5 mM) using both nonlinear least-squares (NLLS) and principal component analysis (PCA). At constant buffer concentration the association constants calculated via NLLS analysis and PCA were found to be, respectively, 3,170 and 3,350 for the 1:1 complex and 100 and 68 for the 2:1 complex. At constant ionic strength the association constants were calculated by NLLS to be 594 for the 1:1 complex and 38 for the 2:1 complex. PCA was used to confirm our model of the solution equilibrium equations used to calculate the association constants by NLLS analysis. In addition, this method yields the spectral line shape and absorptivity of the spectroscopically unresolved 1:1 H₂TCPP⁴⁻:MV²⁺ complex. From this information it was learned that both the absorptivity and the wavelength dependence of H₂TCPP⁴⁻ and the viologen complexes of H₂TCPP⁴⁻ were a function of the alkali metal cation concentration (higher concentrations of cations result in progressively weakened and blue-shifted spectra). Finally, we will present data which support the conclusion of Firman et al. that the spectrum of the H₂TCPP⁴⁻ dimer is weakened and blueshifted by approximately 10 nm relative to the porphyrin monomer in aqueous solution. We will further show that some organic ions induce H₂TCPP⁴⁻ dimerization and some do not; moreover, we will present experimental evidence demonstrating that the data cannot be explained as a simple effect of the ionic strength.