In Section I, we report the generation and a mechanistic study of t-butyl exo-methylene-substituted 2,3-dimethylene-2,3-dihydrofurans and a thiophene analogue. The results not only unambiguously sup- port the previous proposed two-step mechanism for the dimerization of 2,3-dimethylene-2,3-dihydrofuran (1), they also provide us with two of the most unreactive o-quinodimethanes, namely, 2-methylene-3-t-butylmethylene-2,3-dihydrofuran (2), and 2,3-di-t-butylmethylene-2,3-dihydrofuran;In Section II, the preparation and the dimerization of a series of alkyl ring-substituted and carbomethoxy ring-substituted deriva- tives of 1 are presented. This study concludes that bulky alkyl sub- stituents on the ring of furan monomers do not promote the 4+2 dimerization process;In Section III, a Diels-Alder reaction of 1 and its two deuterated analogues with methyl acrylate reveals the presence of a secondary deuterium isotope effect which strongly supports a stepwise mechanism. This conclusion is further strengthened by the fact that the Diels-Alder reaction of 1 with cis dienophiles proceeds non-stereospecifically. A comparison study of o-xylylene with dienophiles reaches the same conclusion;In Section IV, we present a convenient synthesis of cycloocta- decane from the thiophene trimer of 2,5-dimethylene-2,5-dihydro-;thiophene. This synthesis offers an efficient alternative to existing routes; *DOE Report IS-T-1180. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.