The synchronous concerted and stepwise diradical pathways of the retro-Diels-Alder reaction of isopropylidenenorbornene have been studied theoretically at the B3LYP/6-31G* level. The effect of the isopropylidene substituent on the competition between the two mechanisms was determined by comparing these results to those obtained earlier with norbornene. The ease of internal rotation of the methylene radical center relative to the C-C bond cleavage in the diradical intermediate is compared with experimental data on stereoselectivity in the retro-Diels - Alder reaction of stereospecifically dideuterated isopropylidenenorbornene. Experimental and theoretical evidence indicates that the lifetime of the diradical is long enough for the stereochemical scrambling in the thermal retro-Diels-Alder reaction.