The thermal rearrangement reactions of cis-pinane, 1, and trans-pinane, 2, into beta-citronellene, 3, and isocitronellene, 4, have been investigated using ab initio multiconfigurational CASSCF and CASSCF MP2 calculations. Concerted as well as stepwise retro-[2+2]-cycloaddition conversion mechanisms are discussed and the corresponding stationary points along the relevant reaction paths from the bicyclic starting compounds into their acyclic isomers have been optimized. Our calculations show that the stepwise retro-[2+2]-cycloaddition via biradicals is energetically favoured with respect to the concerted mechanism. In the biradical pathways to 3 and 4, it was found that a gauche ring opening of the cyclobutane ring in 1 and 2, respectively, shows significantly lower activation barriers than the competing anti ring opening. With the predicted reaction paths, the calculated activation energies are in very good agreement with experimental values. The reaction mechanisms can explain the differences in the reactivity of 1 and 2, as well as the selectivity differences with respect to the formation of 3 and 4, reported in previous kinetic studies.