Diclofenac belongs to a class of drugs called nonsteroidal antiinffammatory drugs. The kinetics and mechanism of oxidation of diclofenac by sodium N-halo-p-toluenesulfonamides viz., chloramine-T and bromamine-T in NaOH medium have been studied at 293 K. Under comparable experimental conditions, reactions with both the oxidants follow identical kinetics with a first-order dependence on each (oxidant)(o) and a fractional-order dependence on each (diclofenac)o and [NaOH]. Activation parameters have been computed. N-hydroxyldiclofenac is identified as the oxidation product of diclofenac. Michaelis-Menten type of mechanism has been suggested. The rate of oxidation of diclofenac is about four-fold faster with bromamine-T when compared with chloramine-T. This may be attributed to the difference in electrophilicities of Cl+ and Br+ ions and also the van der Waal's radii of chlorine and bromine. Plausible mechanism and related rate law have been designed for the observed kinetics. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 3234-3240, 2009