Multiconfigurational wave functions were used to study the inversion processes of bicyclobutane (C4H6) and its isoelectronic congener bicyclodiazoxane (N2O2). The barriers are about 50 (47) and 40 (32) kcal/mol, respectively, as calculated with multireference CI (second-order multireference perturbation theory). Multiconfigurational descriptions of these systems with simpler GVB wave functions were also carried out. Good agreement between GVB and MCSCF is obtained for geometries. The GVB energetics are not reliable, but relative energies obtained at GVB geometries, using higher levels of theory, provide a reasonable representation of the potential energy surface.