Identification of hydronium ion (H₃O⁺) in the interstellar medium involved several reports over half a decade. In 1986, Wooten et al. and Hollis et al. each reported the P(2,1) rotation-inversion transition line in three objects, OMC-1, Sgr B2, and Orion KL. Confirming lines were subsequently identified in 1991 by Wooten et al. and Goicoechea & Cernicharo. It has also been observed in comets, including Halley and Borrelly, as reported by Balsiger et al. and by Nordholt et al., respectively.

Hydronium ion (H₃O⁺) is protonated water. It has a pyramidal structure and is therefore analogous to ammonia (NH₃).