A π−π interaction in the transition state of the benzylation of 1,3-diphenyl-1-propanone, the monobenzylation product of acetophenone, is proposed according to chemical, kinetic and theoretical approaches. Evidence for the existence of this kind of interaction in a transition state has been provided for the first time. The results obtained cannot be explained solely by the increased acidity but by considering the existence of a π-π interaction.