An ion with a positive charge that is mostly localized on a carbon atom. There are two types:

Carbonium ions have five bonds to the carbon atom and a complete outer shell of eight electrons. A simple example is the ion CH₅⁺, which has a trigonal bipyramidal shape. Ions of this type are transient species. They can be produced by electron impact and detected by mass spectroscopy.

Carbenium ions have three bonds to the carbon atom and are planar, with six outer electrons and a vacant p-orbital. Ions of this type are intermediates in a number of organic reactions (for example, in the S_N1 mechanism of nucleophilic substitution). Certain carbenium ions are stabilized by delocalization of the charge. An example is the orange-red salt (C₆H₅)₃C⁺Cl⁻. Carbenium ions can be produced by superacids.