The structures, energetics, and thermodynamic properties of HIO3 isomers (HOOOI, HOOIO, HOIO2, and HIO3) have been computed using CCSD(T)/CBS theoretical method. The spin orbit corrections (SOC) have also been evaluated for the each iodine-containing molecule. The SOC value decreases as the iodine valence increases within the molecule. The results revealed that HOIO2 was the most stable form and that HOOOI was the next most stable isomer. The standard enthalpies of formation at 298 K have been derived from IO + HO2 and OH + OIO dissociation pathways. At the CCSD(T)/CBS level on B3LYP geometries, the recommended ΔfH°298K is (−95.4 ± 0.3) kJ mol−1 for HOIO2, and its computed S°298K and Cp (300 K) values are respectively 339.12 and 75.28 J mol−1 K−1. Further, monohydrated and dihydrated complexes of iodic acid have been investigated using DFT and wavefunction methods. Three and six mono- and di-hydrated complexes, respectively, have been identified. The hydrogen bonded complexes (HOIO2_1wa and HOIO2_2wa) are the lowest lying structures. The thermodynamics of dihydrates have been calculated from the total hydration of iodic acid (HOIO2 + 2H2O). Mono- and di-hydration of iodic acid is a favored process at tropospheric and ambient conditions with the formation of HOIO2_1wa and HOIO2_2wa at T ⩽ 310 K. The positive Gibbs free energies at high temperatures indicate that gas-phase iodic acid would not be present in a hydrated form inside the nuclear containment building of a pressurized water reactor.