The stereochemistry of the Diels-Alder reaction of 1,2-dichloro-3,3-difluorocyclopropene (5a) to 1,4-diphenyl-1,3-butadiene (6) and 1,3-diphenylisobenzofuran (7) was unambiguously established by X-ray structure determination. In all cases known so far, tetrahalogenocyclopropenes add exo to open-chain dienes and furans. The previously reported exo-addition product (2a) of l-bromo-2-chlorocyclopropene (5b) to 7 allows assignments of the stereochemistry of other additions of 5b to furans. exo-Addition usually predominates, but in some cases endo- adducts are also formed. This contrasts with reports in the literature that 5b adds preferentially endo to open-chain dienes.