Following our investigations on hydrogen polyoxides, herein we employed coupled cluster theory in conjunction with Dunning's correlation consistent basis sets and density functional theory to study HOOOOOH (H2O5). The infrared spectra of H2O5 and its three deuterated isotopologues, as well as those of the five single-substituted 18O isotopologues are discussed in detail. Internal valence coordinates were employed to classify the vibrational modes. The Raman activity is reported to help in the identification of hydrogen pentoxide. The suggested enthalpy of formation is ΔHf,298° (HOOOOH) = 1.4 ± 1.5 kcal/mol. This value includes corrections for relativistic and core-valence effects as well as anharmonic corrections to Zero-point energy corrections. © 2013 Wiley Periodicals, Inc.