In chemistry, a quaternary compound is a compound consisting of exactly four chemical elements.

In another use of the term in organic chemistry, a quaternary compound is or has a cation consisting of a central positively charged atom with four substituents, especially organic (alkyl and aryl) groups, discounting hydrogen atoms.^[1]

The best-known quaternary compounds are quaternary ammonium salts, having a nitrogen atom at the centre.^[2] For example, in the following reaction, the nitrogen atom is said to be quaternized as it has gone from 3 to 4 substituents:

${\displaystyle {\ce {R3N + RCl -> R4N+Cl-))}$

Other examples include substituted phosphonium salts (R₄P⁺), substituted arsonium salts (R₄As⁺) like arsenobetaine, as well as some arsenic-containing superconductors.^[3] Substituted stibonium (R₄Sb⁺)^[4] and bismuthonium salts (R₄Bi⁺) have also been described.^[5]