The substitution reactions of XC6H4COCl [X = 2-, 3-, or 4-CH3; 2-, 3-, or 4-CH3O; 2-, or 4-I; or 2-, 3-, or 4-NO2] and YC6H4COONa [Y = 2-, 3-, or 4-CH3; 2-, 3-, or 4-CH3O; 2-I; 4-NO2; or H] in a two-phase H2O/CH2Cl2 medium using pyridine-1-oxide (PNO) as an inverse phase transfer catalyst were investigated. In general, the kinetics of the reaction follows a pseudo-first-order rate law, with the observed rate constant being a linear function of the concentration of PNO in the water phase. In contrast to other analogous reactions, the hydrolysis reaction of 2-, 3-, or 4-NO2C6H4COCl in H2O/CH2Cl2 medium is catalyzed considerably by PNO and reaches an equilibrium. In the PNO-catalyzed reaction of XC6H3COCl and XC6H4COONa in H2O/C2H2Cl2 medium, the order of reactivities of XC6H4COCl toward reaction with PNO in CH2Cl2 is 2-IC6H4COCl > 4-IC6H4COCl > (C6H5COCl, 3-CH3OC6H4COCl) > 3-CH3C6H4COCl > (2-CH3C6H4COCl, 4-CH3C6H4COCl) > 4-CH3OC6H4COCl > 2-CH3OC6H4COCl. Combined with the results of other analogous reactions, good Hammett correlations with positive reaction constant were obtained for the meta- and para-substituents, which supports that the XC6H4COCl-PNO reaction in CH2Cl2 is a nucleophilic substitution reaction.