Previous studies have indicated that acid-washed pyrite is inert toward aqueous U(VI) under most pH conditions at ambient temperature, even though insoluble UO 2 is the thermodynamically predicted product for the reduction of U(VI) by pyrite. Considering the exothermic nature of nuclear waste, the interaction between uranyl nitrate/acetate and natural pyrite was studied at temperatures ranging from 25 degree celsius to 85 degree celsius and at pH values between-4.0 and-9.5, to simulate the scenarios encountered in a high-level radioactive waste repository. The results revealed that the reactivity of pyrite toward aqueous U(VI) significantly increased with rising temperature, and the reaction could be described by a pseudo-first-order kinetic equation. Activation energies were calculated to be 79.4 +/- 10.6 and 45.7 +/- 3.8 kJ & sdot; mol(- 1) for the reduction of uranyl nitrate, and 78.2 +/- 5.2 and 42.2 +/- 5.8 kJ & sdot; mol(-1) for the reduction of uranyl acetate, at pH values of-4.5 and-5.0, respectively. These values indicate that the reaction is controlled by the surface chemical processes. In addition, the complete reduction of aqueous U(VI) to UO 2 product was first observed for the reactions at pH-4.0 to-5.5 when the temperature was >= 75 degree celsius. Conversely, non-stoichiometric UO 2+x (s) (0 < x <= 0.67) was found at lower temperatures within the same pH range, and also at 85degree celsius with a reaction pH of-9.5. Moreover, the ratio of reduced U (IV/V) on pyrite surfaces increased gradually over time, indicating that reaction time played a significant role in the reduction products. The findings are essential not only for the safety assessment of the high-level radioactive waste repository, but also for understanding the metallogenic mechanism of U(IV)-bearing ore deposits under the relevant anaerobic and hydrothermal conditions.