The substitution reactions of XC6H4COCl (X = 2-F, 3-F, 4-F, or -(CH3)(3)C) and YC6H4COONa (Y = 2-F, 3-F, 4-F, or H) in a two-phase H2O/CH2Cl2 medium using pyridine I-oxide (PNO) as an inverse phase transfer catalyst were investigated. Under suitable reaction conditions, the kinetics of the reaction follows a pseudo-first-order rate law, with a constant observed rate being a linear function of the concentration of PNO in the water phase. The order of reactivities of XC6H4COCl toward reaction with PNO is 2-FC6H4COCl > 3-FC6H4COCl > 4-FC6H4COCl > C6H5COCl > 4(CH3)(3)CC6H4COCl. The order of reactivities of YC(6)H(4)COlO(-) ions toward reaction with 1-(benzoyloxy)pyridinium (C6H5COONP+) ion is C6H5COO- > 3-FC6H4COO- > 4-FC6H4COO- > 2-FC6H4COO-. For the water phase with pH > 12, the reaction is complicated by the competitive reaction of XC6H4COCl and OH- ion in the organic phase.