B3LYP/6-31G* calculations have been performed on the Cope rearrangements of 1,5-hexadienes that are substituted with up to four phenyl groups. Experimental activation enthalpies are available for all of the compounds on which the calculations were performed, and the excellent agreement between the computed and the experimental values provides evidence that the calculated geometries of the transition structures and intermediates are reliable. The calculations confirm that, as suggested by the experimental data, phenyl substituent effects on the Cope rearrangement can be either cooperative or competitive. Based on the computed geometries of the transition structures, it is possible to explain why these two different types of substituent effects are observed.