When CO(g) is added to solutions of the cobaltous porphyrin pi cation radical [Co(II)OEP(.)]ClO4, prepared by oxidation of CoOEP by AgClO4 in anhydrous CH2Cl2, room temperature binding of CO to the metal center occurs. Two distinct products result, [(CO)Co(III)OEP]ClO4 and [(CO)(2)Co(III)OEP]ClO4. These compounds exhibit Soret maxima at 366 and 414 nm, respectively. Resonance Raman (RR) spectra measured before the addition of CO are used to characterize the cobaltous porphyrin pi cation radical, and, after the addition of CO, RR spectra obtained by using 363.8 nm excitation confirm the occurrence of the five-coordinate Co(III) porphyrin complex. This complex, along with halide ligated analogs, displays some structure-sensitive vibrational frequencies which suggest that an unusual distortion of the porphyrin core occurs in CH2Cl2 solution.