High Quality Content by WIKIPEDIA articles! 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a polychlorinated dibenzo-p-dioxin (sometimes in short but inaccurately also called dioxin) with a chemical formula C12H4Cl4O2. TCDD is a colorless solid with no distinguishable odor at room temperature. It is usually formed as a side product in organic synthesis and burning.