The oxidation of chlorpheniramine (CPM) an antihistamine agent by diperiodatoargentate(III) (DPA) has been investigated spectrophotometrically both in the absence and presence of osmium(VIII) catalyst in alkaline medium at a constant ionic strength of 0.10 mol dm-3. The oxidation products were identified as (4-chloro-phenyl)-pyridin-2yl-methanol, dimethyl amino acetaldehyde and Ag(I). The stiochiometry was same in both the cases, i.e., [CPM]:[DPA] = 1:2. In both the uncatalysed and catalysed, the order with respect to DPA concentration was unity while the order with respect to CPM concentration was < 1 over the concentration range studied. The rate increased with an increase in OH- ion concentration and decreased with an increase in IO4 - ion concentration. As the concentration of the catalyst, osmium (VIII), increased the rate of reaction also increased. The order with respect to Os(VIII) concentration was found to be unity. The mechanisms proposed and derived rate laws are consistent with the observed experimental kinetics. Kinetic experiments suggest that Ag(H2IO6)(H2O)2 is the reactive species of oxidant and [OsO4(OH)2]2- is the reactive species of