Department of Chemistry, University of Jodhpur, Jodhpur-342 001 Manuscript received 5 May 1981, revised 25 November 1981, accepted 7 January 1982 The oxidation of mandelic acid and nine monosubstituttd mandelic acids by sodium N-chlorobenzenesulphonamide has been studied in aqueous acetic acid and perchloric acid. The reaction is of first order with respect to the concentrations of the oxidant, the hydroxy acid and hydrogen ions. The primary kinetic isotope effect (kH/kD) is 5.11±0.06 at 303 K. The solvent isotope effect [K(H2O)/k(D2O)] is 0.406 at 303 K. The reaction exhibits a reaction constant p+= -2.13 at 303 K. PbSO2NHCI has been postulated as the active oxidising species. The rates were determinedat four temperatures and the activation parameters were evaluated. A mechanism involving transfer of a hydride ion to the oxidant has been suggested.