The discoloration of the food colorant E131 (Patent Blue V) by K2S2O8 and KIO3 was investigated by spectrophotometric method. The discoloration is first order with respect to the food colorant and the oxidants. The discoloration rate constant increases with the increase in initial concentration of the oxidant and temperature; whereas discoloration decreases with the increase in concentration of E131 and chloride ion. In the presence of persulfate, Cu(II) and Fe(II) have a catalytic effect on discoloration but Ni(II) has not. The thermodynamic parameters (Ea, ∆H # , ∆S # , ∆G # ) of the discoloration in the presence and absence of Cu(II) and Fe(II) are calculated.