A semiclassical trajectory approach has been used to study the cis-trans isomerization from the triplet state of butene-2. It is found that use of the full conformational space for the trajectory calculation permits one to demonstrate that a simplified treatment is applicable; namely, once a transition from the triplet to the ground-state singlet has taken place, return to the triplet is prevented by rapid redistribution of the available energy. The method is used to determine the dependence of the isomerization rate on the essential parameters of the system. (C) 2013 Elsevier B. V. All rights reserved.