High-level electronic structure calculations accurately reproduce activation and reaction energies of dihydrogen eliminations from Cyclic C4H6X molecules. In agreement with the original proposal of Wellington, transition states corresponding to a concerted mechanism are found at the CCSD/aug-cc-pVDZ level of theory. The initial distance between the departing hydrogens determines qualitatively the position of the transition state on the reaction path and (to a lesser extent) its energetics. The excellent agreement between the computed and experimental energies makes the present calculations suitable as a benchmark for calibration of lower-level approaches to be employed for larger heterocycles. (c) 2005 Elsevier B.V. All rights reserved.