Ab initio electronic structure calculations, carried out in conjunction with rigorous analysis of the resulting wavefunctions, demonstrate that the electronic factors responsible for the unusual stability of the 1,3-dimethylimidazol-2-ylidene carbene are directly related to the substantial σ-backdonation from the carbenic carbon to the adjacent nitrogen atoms. The π-donation is found to play only a minor role, ruling out the presence of significant stabilization due to ylidic resonance structures. The unusually large proton affinity of the carbene is explained by the extra π-electron stabilization of the corresponding 1,3-dimethylimidazolium cation. The MP2/6-311++G** level of theory is found to be sufficient for accurate predictions of the geometries of the imidazol-2-ylidene carbenes. © 1993 John Wiley & Sons, Inc.