Solution equilibria and optical spectra of several metallooctaethylporphyrin pi-cation radicals have been examined in methylene chloride solution. The several pi-cation radical species, [M(OEP(.))]Y (M = Cu, Ni, Zn, Pd, or VO; Y = ClO4- or SbCl6-), are found to dimerize to form [M(OEP(.))](2)(2+). These dimeric species are characterized by the appearance of a new, strongly concentration-dependent near-infrared absorption band. This band is found in the region of 900-960 nm for all compounds except for the vanadyl complex which absorbs farther to the red at 1375 nm. The equilibrium constants for the dimerization reaction (2[M(OEP(.))](+) (K-D) [M(OEP(.))](2)(2+)) have been evaluated from multiple-wavelength concentration-dependent absorption data. Equimolar solutions of these pi-cation radicals and their analogous neutral M(OEP) derivatives react to form new binuclear species, [M(OEP(./2))](2)(+), in which the single radical electron is delocalized over both porphyrin rings. These new species, for which we suggest the term "mixed-valence" pi-cation radical and which bear a formal relationship to the oxidized special pair of photosynthetic reaction centers, also display a near-infrared absorption band. The near-IR band is however distinctly different from that of the pi-cation radical dimers. Equilibrium constants for the formation of these new species (M(OEP) + [M(OEP(.))](+) (K-MV) [M(OEP(./2))](2)(+)), have also been determined. The values of K-MV are 10-100-fold larger than the analogous dimerization constant K-D. Values of Delta H and Delta S have been obtained for both equilibrium processes. The variations in K-MV and K-D values are largely the consequence of entropy differences that probably result from a much stronger solvation of the pi-cation radical compared to the neutral porphyrin.