In chemistry, hydronium (hydroxonium in traditional British English) is the cation [H₃O]⁺, also written as H₃O⁺, the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H⁺) to the surrounding water molecules (H₂O). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H⁺ and conjugate base.

Three main structures for the aqueous proton have garnered experimental support:

Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form.^{[non-primary source needed]} For this reason, it has been suggested that wherever possible, the symbol H⁺(aq) should be used instead of the hydronium ion.

The molar concentration of hydronium or H⁺ ions determines a solution's pH according to

where M = mol/L. The concentration of hydroxide ions analogously determines a solution's pOH. The molecules in pure water auto-dissociate into aqueous protons and hydroxide ions in the following equilibrium:

In pure water, there is an equal number of hydroxide and H⁺ ions, so it is a neutral solution. At 25 °C (77 °F), pure water has a pH of 7 and a pOH of 7 (this varies when the temperature changes: see self-ionization of water). A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution.

According to IUPAC nomenclature of organic chemistry, the hydronium ion should be referred to as oxonium. Hydroxonium may also be used unambiguously to identify it.^{[citation needed]}

An oxonium ion is any cation containing a trivalent oxygen atom.