Second-order rate constants k₂ for the reactions of various donor- and acceptor-substituted benzhydrylium ions Ar₂CH⁺ with π-nucleophiles in CH₂Cl₂ were determined by laser flash irradiation of benzhydryl triarylphosphonium salts Ar₂CH-PAr₃⁺X^– in the presence of a large excess of the nucleophiles. This method allowed us to investigate fast reactions up to the diffusional limit including reactions of highly reactive benzhydrylium ions with m-fluoro and p-(trifluoromethyl) substituents. The rate constants determined in this work and relevant literature data were jointly subjected to a correlation analysis to derive the electrophilicity parameters E for acceptor-substituted benzhydrylium ions, as defined by the linear free energy relationship log k₂(20 °C) = s_N(N + E). The new correlation analysis also leads to the N and s_N parameters of 18 π-nucleophiles, which have only vaguely been characterized previously. The correlations of log k₂ versus E are linear well beyond the range where the activation enthalpies ΔH of the reactions are extrapolated to reach the value of ΔH = 0, showing that the change from enthalpy control to entropy control does not cause a bend in the linear free energy relationship, a novel manifestation of the compensation effect. A flattening of the correlation lines only occurs for k₂ > 10⁸ M^–1 s^–1 when the diffusion limit is approached.