Second order rate constants for the reactions of carboxonium ions Ar(MeO)CH+ and aldehyde-boron trihalide complexes ArCH=O-->BX(3) with allylsilanes and alkenes have been determined in CH2Cl2 solution. The relative reactivities of these electrophiles are almost independent of the nature of the pi-nucleophiles, and it is possible to give an averaged reactivity order. For the electrophiles PhCH=O-->BF3, PhCH=O-->BCl3, Ph(p-MeOC(6)H(4))CH+, and Ph(MeO)CH+, i.e., for PhCHX(+) with X = F3BO-, Cl3BO-, p-MeOC(6)H(4), MeO relative averaged reactivities 1:5 x 10(2):1 x 10(4):4 x 10(5) (CH2Cl2, -70 degrees C) are determined indicating that the electrophilicities of aldehyde-BHal(3) complexes are only 3-6 orders of magnitude smaller than those of the corresponding carboxonium ions. The alpha-methoxy substituted benzyl cations Ar(MeO)CH+ are 3-16 times more reactive (CH2Cl2, 20 degrees C) than their phenylogous benzhydryl analogues Ar(p-MeOC(6)H(4))CH+.