The catalytic dechlorination of chlorinated ethylenes by 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin cobalt ((TCPP)-Co), a cobalt complex structurally similar to vitamin 1312, was studied. It was found to have superior aqueous-phase dechlorination activity on chlorinated ethylenes (CEs) relative to vitamin B-12. Bimolecular rate constants for the degradation of CEs by (TCPP)Co of 250, 24, 0,24, and 1.5 M-1 s(-1) were found for perchloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (cDCE), and trans-dichloroethylene (tDCE), respectively. Through kinetic analysis, the rate laws for PCE and TCE were determined to be first order in substrate and catalyst, and PCE degradation was shown to be sensitive to the concentration of the titanium citrate bulk reductant and pH. The importance of the Col oxidation state on dehalogenation was studied with UV-vis absorbance spectroscopy, a variety of reducing agents, and cyclic voltammetry. Evidence of chlorovinyl complexes as potential catalytic cycle intermediates was obtained through the preparation of (TPP)Co(trans-C2H2Cl) and the observation of (TPP)Co-(C2HCl2) and (TCPP)Co(C2HCl2) by mass spectrometry, The X-ray crystal structure of (TPP)Co(tranS-C2H2Cl) is reported.