Bonneau et al. (J. Am. Chem. Sec. 1996, 118,3829-3827) have proposed that carbene-olefin pi-complexes can mediate the stereoselectivity of 1,2-dichloroethylidene (1) rearrangements. Computational studies at MP2 and CBS-Q theoretical levels confirm the experimentally observed preference for rearrangement of 1 to give (Z)-1,2-dichloroethylene. Studies of this and related rearrangements have led to a model to explain the origin of this stereoselectivity, Density functional calculations using Becke3LYP/6-31G* theory suggest that carbene-olefin complexes do not exist as intermediates in the cycloadditions of 1 to ethylene or to tetramethylethylene. The experimental results can be explained by an alternative model in which 18.7% of the diazirine precursor produces alkene product without intervention of the carbene, Becke3LYP/6-31G* calculations on the 1,2-shift of hydrogen concerted with loss of nitrogen and on rearrangement of the diazirine to a diazo intermediate show that these processes are not likely to be the alternative pathways.