This paper presents the mechanism and the kinetics of the I2O5 (g) + H2O (g) = 2 HOIO2 (g) reaction. The potential energy surface was explored with the B3LYP and MP2 methods with the aug-cc-pVTZ basis set. The rate constants were computed as a function of temperature (250-750 K) using transition state theory. At the CCSD(T)/CBS level, the rate constants were estimated to be: (k in cm(3) molecule(-1) s(-1)) k(forward)(T) = 3.61 x 10(-22) x T-2.05 exp (-32.3 (kJ mol(-1))/RT) and k(reverse) (T) = 6.73 x 10(-27) x T-2.90 exp (-24.5 (kJ mol(-1))/RT). Implications for atmospheric chemistry and nuclear safety issues are discussed. (C) 2016 Elsevier B.V. All rights reserved.