AbstractThe products stoichiometry and kinetics of the reduction of trichloronitromethane (chloropicrin), bromotrichloromethane, carbon tetrachloride, ethylene dibromide and 1,2‐dibromo‐3‐chloropropane by iron(II) deuteroporphyrinlX, rat liver P‐450PBand P‐450 cam were compared with the reactions of reduced liver microsomes and the whole‐cell modelPseudomonas putida(PpG‐786). The polyhalomethanes in all cases undergo quantitative reductive hydrogenolysis\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm RCX}_n \, + \,2\,{\rm PFe}^{11} \, + \,{\rm H}^ + \to {\rm RCX}_{n - 1} \, + \,2{\rm PFe}^{{\rm III}} \, + \,{\rm X}^ - $$\end{document}The vicinal halides are Quantitatively converted by all P‐450 components to the corresponding olefinsIn contrast to the steric retardation of the rates exhibited by P‐450 cam, the rate constants for the mammalian enzyme were the same as those obtained for the heme in homogeneous solution. Microsomal reactions followed the same general reactivity patterns but on a much compressed scale. The whole‐cell conversions are apparently controlled in part by permeation; thus the microbial dehalogenation processes proceed at much slower rates than the actual chemistry that occurs at the active site. The results establish that the reductase capacities of widely divergent P‐450 cytochromes are consistent with the chemistry and mechanisms established for hemes in homogeneous solutions. Both microorganisms and mammals have the capacity to dehaloge