The molecular structure and thermochemical stability of 40 bicyclic alkenes and imines containing betweenfive and eight heavy atoms has been investigated computationally by high-level G3/B3LYP method. Thesame method was also used to study their building blocks: monocyclic alkenes and imines. The relativestability of isomers, standard enthalpies of formation and strain energies are discussed and compared. Ourresults suggest that molecular mechanics provides an unsatisfactory description of strain energies in bridgeheadalkenes. Some bridgehead alkene and imine species represent structures which are not local minima on thepotential energy surface and are hence unlikely to be isolated. Furthermore, some bicyclic alkenes andimines have open-shell 1,2-diradical character.