. First-order rate constants k₁ for the trapping of various donor- and acceptor-substituted benzhydrylium ions in mixtures of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and water ranging from 50 to 99% HFIP (w/w) were determined by laser flash photolytic generation of benzhydrylium ions from benzhydryl triarylphosphonium salts in these solvents. From these rate constants, we derived the solvent-specific reactivity parameters N₁ and s_N for HFIP/water mixtures as defined by the linear free energy relationship lg k₁(20 °C) = s_N(N₁ + E).