In an earlier publication, we reported that corrinoids catalyze the sequential reduction of CCl4 to CHCl3, CH2Cl2, CH3Cl, and CH4 with titanium(III) citrate as electron donor [Krone, U. E., Thauer, R. K., & Hogenkamp, H. P. C. (1989) Biochemistry 28, 4908-4914]. However, the recovery of these products was less than 50%, indicating that other products were formed. We now report that, under the same experimental conditions, CCl4 is also converted to carbon monoxide. These studies were extended to include FREONs 11, 12, 13, and 14. Corrinoids were found to catalyze the reduction of CFCl3, CF2Cl2, and CF3Cl to CO and, in the case of CFCl3, to a lesser extent, to formate. CF4 was not reduced. The rate of CO and formate formation paralleled that of fluoride release. Both rates decreased in the series CFCl3, CF2Cl2, CCl4, and CF3Cl. The reduction of CFCl3 gave, in addition to CO and formate, CHFCl2, CH2FCl, CH3F, C2F2Cl2, and C2F2Cl4. The product pattern indicates that the corrinoid-mediated reduction of halogenated C1-hydrocarbons involves the intermediacy of dihalocarbenes, which may be a reason why these compounds are highly toxic for anaerobic bacteria.