Thermolysis of methyl 3-phenylpropyl sulfone in the gas phase results in formation of allylbenzene. Activation parameters of Delta H-double dagger 53.5 +/- 1.0 kcal/mol and Delta S-double dagger = -0.7 +/- 1.4 cal/(mol(.)K) were obtained over the range of 490-550 degrees C. Similar measurements with a deuterated analogue show a substantial isotope effect, and a lower activation enthalpy is observed for the formation of styrene from methyl 2-phenylethyl sulfone. Along with high-quality ab initio calculations of activation parameters and kinetic isotope effects, these results indicate that this is the first reported Ei reaction of a simple sulfone.