The acetolysis of a series of bicyclo[n.1.0]alkanes and [n.m.1]propellanes has been studied. The effect of ring strain, caused by changing ring size or introducing a trans-ring fusion, on the rate and products of the reaction has been examined. No correlation was found between rates of acetolysis and strain energy relief, but with the exception of [3.2.1]propellane, there is a rough correlation with ionization potentials. The degree of polarization of the C-C bonds in the presence of a proton correlates very well with reactivity and is a controlling factor for the acetolysis rates. The importance of the energies of unoccupied orbitals with the appropriate symmetry in controlling electron polarization is shown by the large difference in rate of reaction between [3.2.1]-and [4.2.1]propellanes.