Apart from being an efficient catalyst in acidic medium iridium trichloride, was found to change order of the reaction with respect to cerium(IV) sulphate concentrations in the oxidation of p-methoxy benzaldehyde in aqueous acidic medium. Cerium(IV) and organic substrate form complex in the first equilibrium step. This complex decomposes in the rate-determining step when iridium(RI) is not present in the system while in the presence of iridium(III), it gives rise to another complex, which ultimately gives rise to p-methoxy benzoic acid as product of reaction. Order of the reaction in the absence of iridium(III), was found to be one with respect to cerium(IV) sulphate concentrations while the reaction follows first order kinetics at low concentrations becoming to zero order at higher concentrations of the oxidant in the presence of catalyst. Both in the presence and the absence of catalyst, after showing first order kinetics in the beginning reaction tends to become independent of organic substrate at its higher concentrations. Rate of the reaction follows direct proportionality with respect to catalyst concentrations. Change in the ionic strength of the medium or concentrations of hydrogen ions, cerium(M) and acetic acid have no effect on the rate. Interestingly with fair degree of accuracy, the rate of un-catalyzed path may be calculated graphically with the help of the intercept of the catalyst graph. Product of oxidation was identified to be anisic acid. Energy of activation, free energy of activation and entropy parameters were calculated. (C) 2008 Elsevier B.V. All rights reserved.