Polychlorodibenzo-p-dioxins (chlorodioxins) appear most frequently by heating of chlorophenols in alkaline solution specially when producing organic derivatives. Consequently chlorodioxins may contaminate several industrial chemical products largely used, for instance, as pesticides. Occasionally, chlorodioxins are also synthesized from chlorophenols or their derivatives in situ during industrial processes. Chlorodioxins are the cause of chloracne and chick edema which killed thousands of broiler chickens in the U.S.A. Furthermore chlorodioxins seem to be foetotoxic possible even for man. The present paper is the first part of our work. It is a review of the chemical and toxicological properties of chlorodioxins. We hope to publish later a review of analytical methods and the results of our analyses.