The H5O2+ ion has been studied in chlorocarbon, benzene, and weakly coordinating anion environments to bridge the gap between the gas-phase and traditional condensed-phase investigations. Symmetrical cations of the type [ H5O2+center dot 4Solv] are formed via H-bonding with the terminal O-H groups. In the infrared spectrum, the nu sOH and nu asOH vibrations behave in a manner similar to those of common water molecules: the stronger is the H-bonding interaction with the surroundings, the lower is the frequency shift. A consistent pattern of IR bands from the central O-H+-O group is identified, regardless of the strength of the interaction of H5O2+ with its environment. Three intense bands develop: a ( 860-995 cm(-1)), b ( 1045-1101 cm(-1)), and c ( 1672-1700 cm(-1)), as well as two weak bands, d ( similar to 1300 cm(-1)) and e ( similar to 1400-1500 cm(-1)). These fingerprint bands are highly characteristic for vibrations of O-H-O group irrespective of formal charge. They are seen in symmetrical proton disolvates of the type L-H+-L, where L is an O-atom donor ( alcohol, ether, ketone, phosphate, etc.), and in [ A-H-A](-) acid salts ( A(-)) oxyanion). The commonality is equivalency of the two O-atoms, a short (OO)-O-... distance ( ca. 2.40 angstrom), and a flat-bottomed potential well for the bridging proton, that is, a short, strong, low-barrier H-bond. Assignments for bands a-e are suggested in an attempt to resolve inconsistencies between experimental and calculated data.