Protonated dihydrogen trioxide (HOOOH) has been postulated in various forms for many years. Protonation can occur at either the terminal (HOOO(H)H+) or central (HOOH(OH)(+)) oxygen atom. However, to date there has been no definitive evidence provided for either of these species. In the current work we have employed ab initio methods, CCSD(T) and MP2, with a large basis set (6-311++G(3df,3pd)) to determine the relative stabilities of these species. It is shown that the terminally protonated species is strongly favored relative to the centrally protonated species (Delta E = 15.8 kcal/mol, CCSD(T)//MP2). The mechanism of formation of HOOO(H)H+ was determined to occur with a low barrier with the H3O+ occurring in a thermoneutral reaction (Delta E = -0.3 kcal/mol, CCSD(T)//MP2). Although HOOO(H)H+ exists as a stable intermediate, it is extremely short-lived and rapidly decomposes (Delta E* = 8.6 kcal/mol, MP2) to H3O+ and O-2((1)Delta(g)). The decomposition reaction is stabilized by solvent water molecules. The short-lived nature of the intermediate implies that the intermediate species can not he observed in O-17 NMR spectra, which has been demonstrated experimentally.