Measurements of ∆𝑉 ‡ are a valuable tool to distinguish different reaction mechanisms. A notable example is the case of several Diels-Alder reactions whose activation volumes have given evidences in favor of a concerted one-step mechanism over the competing two-step mechanism. Another exemplary case of competing mechanisms is revealed by the illuminating study of the effect of pressure on the thermal dimerization of 1,3-cyclohexadiene by Klärner et al in 1986. At 1 atm, thermal dimerization of 1,3-cyclohexadiene yielded five reaction products-two [4+2] cycloadducts of the endo and exo configurations, two [2+2] cycloadducts of the syn and anti configurations, and one [6+4]-ene adduct (Figure ). It was also observed that, under pressure up to 7 kbar, the reactions producing all five products reactants transition state product