The cis-and trans-annulated isomers of 8-(N-pyrrolidyl)bicyclo[4.3.0]nona-3,7-diene show different propensities for the retro Dieis-Alder fragmentation following electron impact ionization. Molecular ions of the cis-annulated isomer decompose predominantly via the retro Diels-Alder reaction to give [C9Hl3NIf' fragments of the appearance energy ( A E ) = 8.45 f 0.05 eV and critical energy E, = 133 f 8 kJ mol-'. The trans-annuiated isomer gives abundant [M -HI+ (AE = 9.34 f 0.08 eV) and [M -C&]+' fragments, in addition to [C9H13N]+' ions of A E = 8.98 * 0.05 eV and E, = 181 *8 k J mol-'. The ionization energies (ZE) were determined as ZE& = 7.07 f0.05 eV and Z E , , = 7.10 f 0.06 eV. The stereochemical information is much less pronounced in unimolecular decompositions of long-lived (metastable) molecular ions which show very similar fragmentation patterns for both geometrical isomers. Nevertheless, the isomers exhibit different kinetic energy release values in the retro Dieis-Alder fragmentation; To, = 3.8 f 0.3 and 4.8 f 0.2 kJ mol-' for the cis and trans isomer respectively. Topological molecular orbital calculations indicate that the retro Dieis-Alder reaction prefers a two-step path, with a subsequent cleavage of the C(5)-C(6) and C(l)-C(2) bonds. The open-ring distonic intermediate represents the absolute minimum on the reaction energy hypersurface. The cleavage of the C(l)-C( 2) bond is the rate-determining step in the decomposition of the cis isomer, with the critical energy calculated as 137 k J mol-'. The cleavage of the C(5)-C( 6) bond becomes the rate-determining step in the trans-annulated isomer because of stereoelectronic control. The difference in the energy barriers to this cleavage in the isomers (AE = 95 kJ mol-') provides a quantitative estimate of the magnitude of the stereoelectronic effect in cation radicals.