An adduct (from the Latin adductus, "drawn toward") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components, with formation of two chemical bonds and a net reduction in bond multiplicity in at least one of the reactants. The resultant is considered a distinct molecular species. Examples include the adduct between hydrogen peroxide and sodium carbonate to give sodium percarbonate, and the addition of sodium bisulfite to an aldehyde to give a sulfonate.

Adducts often form between Lewis acids and Lewis bases. A good example would be the formation of adducts between the Lewis acid borane and the oxygen atom in the Lewis bases, tetrahydrofuran (THF) or diethyl ether: BH3•THF, BH3•OEt2. Compounds or mixtures that cannot form an adduct because of steric hindrance are called frustrated Lewis pairs.

In the formulae of addition compounds or compounds which can formally be regarded as such, including clathrates and multiple salts, the formulae of the component molecules or entities are cited in order of increasing number; if they occur in equal numbers, they are cited in alphabetical order in the sense of Section IR-4.4.2.2. In addition compounds containing water, the water remains conventionally cited last. However, component boron compounds are no longer treated as exceptions.