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 cm3 molecule−1 s−1) kforward(T) = 3.61 × 10−22 × T2.05 exp (−32.3 (kJ mol−1)/RT) and kreverse (T) = 6.73 × 10−27 × T2.90 exp (−24.5 (kJ mol−1)/RT). Implications for atmospheric chemistry and nuclear safety issues are discussed. © 2016