Summary. The photoelectron spcctrum of the title compound 2 is discussed and compared with the results of MO-calculations for the parent unsubstitutcd hydrocarbon 1. The ground-statc of 2+ is SE" which is ]ah?a-Teller ur) active. Assuming the e' C-C-stretch to be responsible for breaking its degeneracy, satisfactory agreement between experimental and theoretical band shape is obtained. The latter is calculated from the diabatic two-state approximation using MIND01 3-SCF results for the ]?=stabilization energies. Comparison with Hydbwg studies on 1, performed by other huthors, yields further evidencc for the A; -(not W -) symmetry of the Rydbcrg statcs.The next two obsorvcd higher lying states of 2f are a E and 9A;. the former also being IT-active.Thcoretical calculations of the corresponding band shapo point toward the e'-ring stretching mode causing instability, an outcome reminiscent of the '37-groundstate of cyclopropane-ion. The encrgetic diffcrcncc betwcen the latter state and the *=-state of 2f is discussed using a simple MO-model with methylidenc and ethylidene carhne as interacting units. The results suggest that the unsaturatcd cnrbene is strongly stabilized by hyperconjugation. The chemistry of the two carbenes is compared on this basis.