Radical cations are denoted ${\displaystyle M^{+\bullet }}$. Salts of these species have been isolated in the cases of dibenzocyclooctatetraene, various tertiary amines, and some polymethylated derivatives of azulene.^[1] Radical cations, like radical anions, have one unpaired electron, i.e. they are paramagnetic.

Radical cations appear prominently in mass spectrometry.^[2] When a gas-phase molecule is subjected to electron ionization one electron is abstracted by an electron in the electron beam to create a radical cation M^+.. This species represents the molecular ion or parent ion. A typical mass spectrum shows multiple signals because the molecular ion fragments into a complex mixture of ions and uncharged radical species. For example, the methanol radical cation fragments into a methenium cation CH+3 and a hydroxyl radical. In naphthalene the unfragmented radical cation is by far the most prominent peak in the mass spectrum. Secondary species are generated from proton gain (M+1) and proton loss (M-1).

Some compounds containing the dioxygenyl cation can be prepared in bulk.^[3] Many transition metal complexes are radicals and cationic, e.g. [MCl₄]^2- (M=Mn, Fe, Co, Ni). Such species are so pervasive that they are rarely discussed in the context of radicals.

Radical cations figure prominently in the chemistry and properties of conducting polymers. Such polymers are formed by the oxidation of heterocycles to give radical cations, which condense with the parent heterocycle. For example, polypyrrole is prepared by oxidation of pyrrole using ferric chloride in methanol:

n C₄H₄NH + 2 FeCl₃ → (C₄H₂NH)_n + 2 FeCl₂ + 2 HCl

Once formed, these polymers become conductive upon oxidation.^[4] Polarons and bipolarons are radical cations encountered in doped conducting polymers.

Carbocations are diamagnetic ions with a positively charged carbon atom. Carbocation are further classified in two main categories according to the coordination number of the charged carbon: three in the carbenium ions and five in the carbonium ions. Among the simplest carbocations are the methenium CH+3 (a carbenium ion) and methanium CH+5 (a carbonium ion).^[5]

• Radical anion