Gamma radiation induced decoloration and degradation of aqueous solution of Reactive Red 120 dye (RR-120) have been investigated under different experimental conditions. Rate constants for the reaction of hydrated electron and hydroxyl radical with RR-120 were determined to be 1.2×10 10 and 7.9×10 9 mol −1 dm 3 s −1 , respectively, by pulse radiolysis technique. The decoloration and degradation efficiency were measured in terms of % decoloration and % TOC, respectively. Decoloration was observed to be most efficient under reducing condition, where the radiolytic yield for the decoloration of dye was determined to be 0.14μmol/J. The extent of decoloration for both aerated and oxygen saturated solution was almost identical, whereas it decreased in N 2 O saturated solution as well as N 2 saturated solution. For a solution having 10.56μg/ml total organic carbon (TOC) at a dose of 3kGy, 48% mineralization takes place in oxygen saturated solution whereas under aerated condition same was observed to be lowered to 38%.