3-Ethylidene-2-methylene- (12), 2-ethylidene-3-methylene- (13), and 2,3-diethylidene- (14) 2,3-dihydrofurans were prepared by fluoride by fluoride-induced 1,4-conjugative elimination of trimethylsilyl acetate from the appropriate precursors. The H-1 NMR spectra of these furan-based o-quinodimethanes were obtained and the dimerization products of each were studied. It was found that a methyl group at the 3-methylene position retards the rate of dimerization which is consistent with the previously proposed dimerization mechanism, the two-step mechanism involving rate-determining formation of a diradical intermediate followed by rapid cyclization of the diradical. Structures were assigned to 6 dimers from 12, 7 dimers from 13, and 5 dimers from 14. Most of these dimers are [4 + 4] and [4 + 2] cyclo dimers, but one, 44, is an intramolecular disproportionation product derived from the diradical proposed in the dimerization of 13. Identification of dimer 44 provides additional support for the two-step diradical mechanism. From analysis of the stereochemistry and regiochemistry of the dimers, it is concluded that both cisoid and transoid diradical intermediates are formed by cisoid and transoid encounters of two monomer molecules. Also, from analysis of the products it is concluded that the regiochemistry of the cyclization of the diradical intermediates is controlled mainly by the interaction of the active sites of the furan moieties in the cyclization step; the initial conformation of the intermediate is not important.