The oxidation of isoniazid by bromate was investigated kinetically under pseudo-first-order conditions in acid medium. The reaction was accelerated by the hydrogen ion due to the various protonation equilibria of both the reactants. The mechanism involves diprotonated isoniazid as the active species which forms two complexes with the diprotonated and unprotonated oxidant. The complex formed between the diprotonated oxidant and the protonated -NH2 group of hydrazide moiety of isoniazid decomposes into isonicotinic acid. Formation of another stable ion pair complex decreases the rate of reaction. The UV-vis examination of the solutions support the formation of both protonated isoniazid and the complexes. A probable mechanism on the basis of kinetic results involving formation of an acyl diimide of the isoniazid was proposed.