The water cation (H₂O⁺) was identified by Ossenkopf and 100 co-authors using the Herschel HIFI instrument. It was found in several star-forming regions: DR21, Sgr B2, and NGC6334. Both ortho and para forms have observed, as reported by Schilke and co-workers. H₂O⁺ has also been observed toward Orion KL by Gupta and co-workers and toward the extragalactic sources NGC4418 and Arp 220 by González-Alfonso and co-workers.

Water cation is also called oxidaniumyl, with the ium indicating that it is a cation and the yl indicating that it is a open-shell radical. The structure of the water cation is similar to that of water, with bond angle and bond lengths that are both slightly larger. The ionization energy of water is 12.621 eV, which is about 1 eV less than the energy (13.618 eV) required to ionize atomic O by removing a 2p electron from the 2p2 pair. The singly occupied (2p-like) orbital of H2O+ is shown in the figure.