Any member of a class of organic molecules with positive charges localized at a carbon atom.

Certain carbonium ions can be prepared in such a way that they are stable enough for study; more frequently they are only short-lived forms (intermediates) occurring during chemical reactions.

Carbonium ions are, in fact, one of the most common classes of intermediates in organic reactions, and knowledge of the structures and properties of these substances is fundamental to understanding reactions in which they occur.

Many of these reactions are of synthetic, biochemical, or industrial importance.