From the pressure dependence of cyclodimerization of chloroprene, it has been concluded that volumes of activation may be useful for the distinction between competing concerted and stepwise cycloadditions. Accordingly, one of the [4 + 2] cyclodimers, 1,4-dichloro-4-vinylcyclohexene (3), should be formed in a stepwise Diels-Alder reaction involving a diradical intermediate analogously to the [2 + 2] cyclodimerizations leading to cis- and trans-1,2-dichloro-1 ,2-divinylcyclobutane (5 and 6). A stereochemical analysis using (E)-1-deuteriochloroprene (1-D-E shows, indeed, a nonstereospecific course for the formation of the formal Diels-Alder adduct 4 (cis:trans ratio of 59.6:40.4), as expected for a stepwise process, and confirms the conclusion drawn from pressure dependence. The mechanism of the Diels-Alder dimerization of 1,3-butadiene (13) is elucidated by an investigation of the effect of pressure and the stereochemistry. The activation volume found for the Diels-Alder dimerization leading to 4-vinylcyclohexene (16) turned out to be substantially lower than that found for the competing [2 + 2] cyclodimerization leading to trans-1 ,2-divinylcyclobutane (17) (Delta Delta V* = -13.3 cm(3)/mol). The dimerization of (Z,Z)-1,4-dideuterio-1,3-butadiene (13-1,4-D-Z) shows only 3% loss of stereochemistry in the formation of 16-D at 1 bar and <1% at 6.8-8 kbar (cis:trans ratios of 97:3 and >99:<1, respectively). These findings provide good evidence for a stereospecific pericyclic Diels-Alder mechanism competing with a small amount of nonstereospecific stepwise reaction which is almost completely suppressed by high pressure. Volumes of activation and reaction are calculated for the Diels-Alder reaction of ethene with 1,3-butadiene (13) and the various dimerization pathways of 1,3-butadiene (13) by a Monte Carlo computer simulation using the model of hard spheres to describe the various ground and transition structures. The good agreement between calculated and experimental data shows that volumes of activation and reaction cannot be explained by properties of single molecules. They require consideration of configurational effects (e.g., the different packing of cyclic and acyclic states) for their interpretation. Thus, activation volumes can provide important information on transition-state geometries.