A kinetic and mechanistic study of the 4+4 dimerization of 2,3-dimethylene-2,3-dihydrofuran (1) has been carried out based on the secondary deuterium kinetic isotope effects (2(DEGREES) D KIE), substituent effects and solvent effects. The 2(DEGREES) D KIE was studied by measuring the rate constants, k(,1), k(,2), k(,3) and k(,4), respectively, for the 4+4 dimerization of 1, 2-methylene-3-dideuteriomethylene-2,3-dihydro- furan (2), 2-dideuteriomethylene-3-methylene-2,3-dihydrofuran (3) and 2,3-bis(dideuteriomethylene)-2,3-dihydrofuran (4). The relation among k(,1) - k(,4) at -30.0(DEGREES)C was found to be k(,2)/k(,1) = k(,4)/k(,1) =1.86 and k(,1) = k(,3). This result and the insignificant solvent effects that have been observed lead to the conclusion that the 4+4 dimerization of 1 proceeds via stepwise mechanism involving a diradical intermediate;2,3-Dimethylene-2,3-dihydrobenzo b furan (5) was synthesized by the flash vacuum pyrolysis of 2-methyl-3-benzo b furylmethyl benzoate. The chemical reactions of 5, which were investigated in this study, include thermal dimerization reactions and Diels-Alder reactions. The dimerization reaction of 5 gave both 4+2 and 4+4 dimers;The formation of cyclic peroxides from the reactions of 1 and its 5-methyl and 5-tert-butyl derivatives with triplet oxygen was;investigated. Thermolysis of a cyclic peroxide gave dialdehyde, hydroxy aldehyde and diol as the products; *DOE Report IS-T-1186. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.