The different photochemistry of the title compounds (reactions of the endocyclic 1,5-dlene moiety versus a photochemical [1,3]-OH shift) can be explained assuming an initial isomerization of the exocyclic double bond. MNDO/CI calculations of the potential energy curves and nonadiabatic couplings for the rotation of this bond showed that the 90° twisted conformation can easily be reached. For germacrol the lowest excited state has a zwitterionic character which is favourable for a planar photochemical [1,3]-OH shift. For germacrene, this polarized state is strongly coupled to two diradicalar states. In these twisted diradicalar states a redistribution of the charges in the endocyclic double bonds is found which is eminently suited for intramolecular bond formation.