In chemistry, a lyonium ion is the cation derived by the protonation of a solvent molecule. For example, a hydronium ion is formed by the protonation of water, and  is the cation formed by the protonation of methanol.
Its counterpart is a lyate ion, the anion formed by the deprotonation of a solvent molecule.
Lyonium and lyate ions, resulting from molecular autoionization, contribute to the molar conductivity of protolytic solvents.