Chemical reactivity, biotransformation and toxic effects of chlorinated aliphatic compounds are decisively determined by type and number of chlorine substitutions. In saturated compounds, chlorine substitution exerts a destabilization; metabolic reactions are free radical formation and dechlorination or dehydrochlorination, respectively, under formation of olefins. In saturated alkenes, Cl-substitution leads to a stabilization of the olefinic double bond. Biotransformation of this class of compounds starts with epoxidation. The epoxides are prone, among other reactions, to intramolecular rearrangement; more stable aldehydes or acid chlorides are formed. In the series of chlorinated ethylenes, the rearrangement mechanisms have been studied systematically in vitro and in vivo. As judged from the identified metabolites there is accordance in vitro and in vivo, with one important exception: trichloroethylene. The only formation of chloral in vivo may be due to a catalysis by the iron in P450. In a modified Ames test system the unsymmetrically substituted chlorinated ethylenes are found mutagenic (tri-,1,1-dichloroethylene and vinyl chloride), whereas symmetrically substituted molecules are inactive. The reason for this is seen in the high instability of the unsymmetric molecules caused by the electron withdrawal effect of chlorine.