Catalytic dehalogenation of chlorinated and fluorinated ethylenes by (PR3)3RhCl complexes is described. The C-Cl and C-F bonds are activated by the catalyst in the presence of triethylsilane (Et 3SiH) or dihydrogen (H2). Spectroscopic studies, in addition to substrate preference, indicate that rhodium hydride species are important intermediates. Kinetic parameters and product distribution for dehalogenation reactions were determined using NMR spectroscopy. Evidence for sequential chlorine removal was obtained, and the rates of dehalogenation were found to increase with decreasing halogen content. It was also shown that this catalytic system has a preference for sp2over sp3- hybridized carbon-halogen bonds. Dechlorination using (PPh3) 3RhCl and either H2 or Et3SiH supports an insertion/β-chloride elimination mechanism; however, the two systems display distinct differences. On the basis of these differences, the dominant pathway for Et3SiH is proposed to involve rhodium(I), while the H2 system is proposed to primarily involve rhodium(III). This is supported with isotopic labeling studies using D2, Et3SiD, and (PPh3)3RhD, which yield different stereochemistry of dechlorinated products. With D2, only products consistent with Syn-β-chloride elimination were observed, while with Et3SiD and (PPh3)3RhD both syn- and anti-β-chloride elimination products were observed. In addition, NMR spectroscopic evidence of different hydride intermediates in the H2 and Et3SiH systems was obtained. Different pathways for dehalogenation with Et3SiH and H2 is further supported by the observation of 1,2-addition (hydrogenation) products using H2 and the lack of 1,2-addition (hydrosilylation) products using Et3SiH.