Reaction barriers were calculated by using ab initio electronic structure methods for the reductive dechlorination of the polychlorinated ethylenes: C2O4, C2HCl3, trans-1,2-C2H2O2, cis-1,2-C2H2O2, 1,1-C2H2O and C2H3Cl. Concerted and stepwise cleavages of R-Cl bonds were considered. Stepwise cleavages yielded lower activation barriers than concerted cleavages for the reduction of C2O4, C2HCl3, and trarrs-1,2-C2H2Cl2 for strong reducing agents. However, for typical ranges of reducing strength concerted cleavages were found to be favored. Both gas-phase and aqueous-phase calculations predicted C2Cl4 to have the lowest reaction barrier. Additionally, the reduction of C2HCl3 was predicted to show selectivity toward formation of cis-1,2-C2HCl2 center dot over the formation of trans- 1,2-C2HCl2 center dot, and 1,1-C2HCl2 center dot radicals.