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 x 10¹⁰ and 7.9 x 10⁹ mol^-1 dm³ 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 0.14 µmol/J. The extent of decoloration for both aerated and oxygen saturated solution was almost identical, whereas it decreased in N₂O saturated solution as well as N₂ saturated solution. For a solution having 10.56 µg/ml total organic carbon (TOC) at a dose of 3 kGy, 48% mineralization takes place in oxygen saturated solution whereas under aerated condition same was observed to be lowered to 38%.