The kinetics and mechanism of Os(VIl1) utalysed oxidation of some suhstituted transcinnnmic acids by chlonmine-T in presence of alkaline medium has been investigated. The order with respect to [CAT] is zero and is lOsO ] one. The reaction is fractional order in [substrate]. The rate dependace on [OH'] is inverse t&t order. Both the electron-relmsing snd withdrswing substituents fscilitste the reaction rate. The observed rate constants for substituents are plotted against Hammett constant o, a non-linear, 'V' shaped curve is obtained. The electronwithdrswing substituents fall on one side of the curve hsving s negative P value, and electron-t&u@ substituents fall on the otherside with positive P value. The break in the Hammett plot is due to change in the nature of transition state. The mechanism proposed and the derived rate law are in contirmity with the observed results. The kinetics of Os(VIII)-catalysed oxidation of cinnamic acid by chlorate' and Ce(IV)* have already been reported. But there is no systematic kinetic report on the Os(VII1) catalysed oxidation of cinnamic acid by N-halocompounds. This promted as to undertake the title for investigation. Many oxidation reaction? are carried out using chloramine-T (CAT). The reaction has been studied under different experimental conditions, in the presence of sodium hydroxide using water as a solvent. The reaction does not take place in the absence of the catalyst. Osmium tetroxide is found to be catalysing the reaction. EXPERIMENTAL Reagents. All the cinnamic acids used were of Fluka sample. CAT, NaOH and 0~0~ were of AR grade. Kinetic runs were followed by estimating CAT by standard iodometric method. Doubly distilled water was employed second distillation being from alkaline permanganate solution in all glass assembly. Kinetic measuremenn. For kinetic runs, a measured amount of CAB pre-eqtilibrated at 35°C was rapidly added to the thermally equilibrated mixture containing appropriate amounts of the cinnamic acid, alkali, osmium tetroxide and water. The progress of the reaction was monitored for two half lives by iodometric determination of unreacted CAB in a measured aliquot withdrawn at regular intervals of time. The zero order rate constants were reproductive within f3 46.