Diels-Alder dimerizations of a series of substituted cyclopentadienes have been studied. Experimentally, preparation of 1-oxaspiro[2.4]hepta-4,6-diene results in immediate formation of the Diels-Alder dimer with two adducts formed in a ratio of 3:1. Two substituted spiroheptadiene systems gave similar results. Theoretically, transition states for four different isomers were located leading to the four dimer minima of 1-oxaspiro[2.4]hepta-4,6-diene. The transition state associated with the lowest of the four energetic barriers leads to the isomer with the same connectivity as the major isomer obtained in the dimerization of 2,2-dimethyl-1-oxaspiro[2.4]hepta-4,6-diene (structure confirmed by X-ray diffraction analysis). Unsymmetric transition states at the HF/6-31G(d) level were predicted for three additional endo species with the energy barriers decreasing in the following order : spiro[2.4] hepta-4, 6-diene > cyclopentadiene > 1-oxaspiro[2.4]hepta-4,6-diene > 1,2-dioxaspiro[2.4]hepta-4,6-diene. The barrier to forming the dimer of spiro[2.4]hepta-4,6-diene is predicted to be approximately 5 kcal/mol greater than that for forming the cyclopentadiene dimer and about 13 kcal/mol greater than the barrier to formation of the dimer of 1-oxaspirol[2.4]-hepta-4,6-diene.