The substitution reactions of 2-, 3- or 4-chlorobenzoyl chloride (2-, 3- or 4-ClC6H4COCl) and 2-, 3- or 4-chlorobenzoate ion (2-, 3- or 4-ClC6H4COO−) in a two-phase H2OCH2Cl2 medium using pyridine 1-oxide (PNO) as an inverse phase-transfer catalyst were investigated. Experimental results strongly support that the reaction of ClC6H4COCl and PNO in organic phase to produce the intermediate, 1-(chlorobenzoyloxy)pyridinium chloride (ClC6H4COONP+Cl−) is the rate-determining step and that the equilibrium distribution of PNO between H2O and CH2Cl2 phases is always maintained. The order of reactivities of benzoyl chlorides toward reaction with PNO is 2-ClC6H4COCl > 3-ClC6H4COCl > 4-ClC6H4COCl > PhCOCl. The order of reactivities of benzoate ions toward reaction with PhCOONP+ ion is (3-, 4-ClC6H4COO−) > 2-ClC6H4COO− > PhCOO−.