The cumyl cation was generated by laser flash photolysis of cumyl tris(4-chlorophenyl)phosphonium tetrafluoroborate in CH2Cl2 and identified by its UV spectrum. From the decay of its absorbance at lambda = 335 nm in the presence of variable concentrations of several nucleophiles with CC double bonds, rate constants for the reactions of the cumyl cation with these pi-nucleophiles were determined. The linear free energy relationship log k(20 degrees C) = s(N + E) (eq 1) was used to calculate the electrophilicity parameter E = 5.74 of the cumyl cation from the rate constants determined in this work and the previously reported N and s parameters of the nucleophilic reaction partners. Substitution of E of the cumyl cation and of the previously reported N and s parameters of alpha-methylstyrene into eq I predicts the temperature-independent rate constant of the addition of the cumyl cation to alpha-methylstyrene (1.2 x 10(8) M-1 s(-1)), which is relevant for the cationic polymerization of alpha-methylstyrene.