Octaethylporphyrin cobalt(II), (OEP)Co-II, oxidizes to form a paramagnetic porphyrin pi cation radical complex, [(OEP)Co-II](+), that is in equilibrium with a diamagnetic dimer, [(OEP)Co-II](2)(2+). Thermodynamic and activation parameters for dissociation of the diamagnetic dimer into paramagnetic monomers (Delta H degrees = 15.9 +/- 0.5 kcal mol(-1), Delta S degrees = 43 +/- 3 cal K-1 mol(-1); Delta H double dagger = 18.7 +/- 0.8 kcal mol(-1), Delta S double dagger = 27 +/- 3 cal K-1 mol(-1)) were determined from the temperature dependence of the H-1 NMR chemical shifts and line widths, respectively.