Abstract A molecular model for the carcinogenicity of carbon tetrachloride has been previously suggested in which an electron is transferred from an enzyme to the CCl 4 molecule resulting in its dissociation into Cl − and the free radical .CCl 3. Cellular damage was attributed to the free radical. In the light of this model we examined the series of one carbon halocarbons CF 4, CCl 4, CBr 4, CI 4, CHCl 3, CHBr 3 and CHI 3 for their carcinogenic activity. The propensities of the halocarbons to produce free radicals by dissociative electron attachment was obtained from calculated potential energy curves and other physical properties, and the ability of the free radicals to abstract hydrogen atoms from the lipid was estimated from the CH bond strengths in the appropriate molecules formed when the free radical combines with the hydrogen atom. Using these two parameters it proved to be possible to assign a toxicity to each of the halocarbons studied which was in agreement with all animal study results for which data were available.