Online spectrophotometry method is employed to Monitor simulated Reactive Brilliant Blue X-BR (RBB X-BR) in aqueous solution in Fenton oxidation process. The effects of initial dosage of FeSO4 and H2O2, pH value, initial concentration of dye and temperature have been studied. The results show that online spectrophotometric method is a quick, feasible and convenient technique to monitor color removal of RBB X-BR in Fenton process. The optimal dosage of H2O2 and pH is 3.529 mM and 3, respectively. The optimal dosage of Fe2+ for color removal is 0.1618 mM. The concentration of initial FeSO4 against the reaction rate constant (k(ap)) for decolorizing is linear correlation as: k(ap) = 0.1354[Fe2+](o) (R-2 > 0.99). The apparent activation energies of reaction is 25.21 kJ mol(-1) (R-2 > 0.99). The intrinsic reaction rate constant of (OH)-O-center dot with RBB X-BR in aqueous solution is 7.396 x 10(10) M-1 s(-1). The molecule structure of RBB X-BR is decomposed and not mineralized by Fenton's reagent. The main intermediate products are 1.2-diacetylenzene and 2.5-diritrobenzoic acid. The probable mechanism of the decoloration of RBB X-BR is also discussed. (c) 2008 Elsevier B.V. All rights reserved.