The oxidation reaction of vanillin with cerium (IV) in perchloric acid medium has been studied spectrophotometrically over the range 5.0 <= 10(3)[Vanillin](T) <= 25.0; 0.3 <= [H+](T) <= 0.8; I = 1.0 mol dm(-3) (NaClO4) and 293 <= T <= 308K. The rate of the reaction has been found to increase with the increase in [H+](T) and [Vanillin](T). The reaction follows first order kinetics with respect to each [Ce(IV)](T), [Vanillin](T) and fractional order with respect to [H+](T). The reaction proceeds in two paths and the values of k(1) and k(2) have been calculated using a suitable rate law. The Delta H-# (kJ mol(-1)) for k(1) and k(2) paths are 3.45 +/- 0.24 & 43.61 +/- 2.49 and Delta S-# (JK(-1) mol(-1)) for k(1) and k(2) paths are -247.81 +/- 0.81 & -141.23 +/- 8.26 respectively. High negative value of activation entropy is indicative of ordered transition state for the reaction. The product of the reaction has been found to be 4-hydroxy-3-methoxybenzoic acid.