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Abstract. C12H4C140 , M r = 305.97, monoclinic, C2/c, a = 14.702 (4), b = 12.886 (4), c = 6.256 (1) A, fl--99.90 (2) °, V= 1168 A, Pobs = 1.72 (flotation), Pcalc = 1.74 g cm -3, Z -=--4. The structure has been determined by direct methods and refined to R = 0.042 for 1863 independent reflections. The molecule is essentially planar. A crystallographic twofold axis bisects a C-C bond and passes through the O atom of the fivemembered furan ring. The two unique C-C1 distances are 1.725 (2) and 1.732 (2) A, the C-O distance is 1.385 (2) A, and the benzenoid ring C-C distances range between 1.366 (2) and 1.404 (2) A. The longest C-C bond distance within the benzenoid rings joins the C atoms to which the C1 atoms are attached. The title compound is closely related in structure to the highly toxic 2,3,7, 8-tetrachlorodibenzo-p-dioxin.