High-precision experimental kinetic isotope effects were measured at natural abundance for the cycloaddition of dichlorocarbene to 1-pentene. These values were compared to isotope effects predicted for the addition of dichlorocarbene to propene at the B3LYP/6-31G* and B3LTP/6-311+G* levels. The results provide unambiguous evidence for a nonlinear attack in which bond formation is more advanced at the unsubstituted carbon of the terminal alkene in the transition state of the cycloaddition. This is the first experimental confirmation of the unsymmetrical, non-least-motion approach of a carbene to an alkene first proposed by Skell over 40 years ago and predicted by quantum mechanical calculations.