Rate constants for the reactions of benzaldehyde-derived iminium ions with C-nucleophiles (enamines, silylated ketene acetals, and enol ethers) have been determined photometrically in CH₃CN solution and used to determine the electrophilicity parameters E of the cations defined by the correlation log k_20°C = s_N(E + N) (Mayr, H.; et al. J. Am. Chem. Soc.2001, 123, 9500–9512). With electrophilicity parameters from E = −10.69 (Ar = p-MeOC₆H₄) to E = −8.34 (Ar = p-CF₃), the iminium ions Ar–CH═NMe₂⁺ have almost the same reactivities as analogously substituted arylidenemalononitriles Ar–CH═C(CN)₂ and are 10 orders of magnitude more reactive than the corresponding aldehydes. The rate constants for the reactions of iminium ions with amines and water in acetonitrile are 10³–10⁵ times faster than predicted by the quoted correlation, which is explained by the transition states which already experience the anomeric stabilization of the resulting N,N- and O,N-acet