In chemistry, an oxonium ion is any oxygen cation with three bonds. The simplest oxonium ion is the  hydronium ion H₃O⁺. Another class of oxonium ions frequently encountered in organic chemistry is the oxocarbenium ions, obtained by protonation or alkylation of a carbonyl group e.g. R-C=O⁺-R' which forms a resonance structure with the fully fledged carbocation R-C⁺-O-R' and is therefore especially stable:

Stable alkyloxonium salts exist; they are extensively used as alkylating agents. For example, triethyloxonium tetrafluoroborate  is a white crystalline solid. It is a powerful ethylating agent. It can be used, for example, to produce ethyl esters when the conditions of traditional Fischer esterification are unsuitable. It is also used for preparation of enol ethers and related functional groups.