In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of catalysis and can act through homogeneous catalysis or heterogeneous catalysis methods depending on the catalyst used. Ionic reactants are often soluble in an aqueous phase but insoluble in an organic phase in the absence of the phase-transfer catalyst. The catalyst functions like a detergent for solubilizing the salts into the organic phase. Phase-transfer catalysis refers to the acceleration of the reaction upon the addition of the phase-transfer catalyst. PTC is widely exploited industrially. Polyesters for example are prepared from acyl chlorides and bisphenol-A. Phosphothioate-based pesticides are generated by PTC-catalyzed alkylation of phosphothioates.

In ideal cases, PTC can be fast and efficient, minimizing the need for expensive or dangerous solvents and simplifying purification Phase-transfer catalysts are "green"—by allowing the use of water, the need for organic solvents is lowered.

Phase-transfer catalysts for anionic reactants are often quaternary ammonium salts. Commercially important catalysts include benzyltriethylammonium chloride, methyltricaprylammonium chloride and methyltributylammonium chloride. Organic phosphonium salts are also used, e.g., hexadecyltributylphosphonium bromide. The phosphonium salts tolerate higher temperatures.

An alternative to the use of "quat salts" is to convert alkali metal cations into hydrophobic cations. Crown ethers are used for this purpose on the laboratory scale. Polyethylene glycols and their amine derivatives are common in practical applications. One such catalyst is tris(2-(2-methoxyethoxy)ethyl)amine. These ligands encapsulate alkali metal cations (typically Na⁺ and K⁺), affording lipophilic cations. Polyethers have a hydrophilic "interiors" containing the ion and a hydrophobic exterior.

Chiral phase-transfer catalysts have also been demonstrated. Asymmetric alkylations are catalyzed by chiral quaternary ammonium salts derived from cinchona alkaloids.

A variety of functionalized catalysts have been evaluated for PTC. One example is the Janus interphase catalyst, applicable to organic reactions on the interface of two phases via the formation of Pickering emulsion.

Quaternary ammonium cations degrade by Hofmann degradation to amines, especially at higher temperatures preferred by process chemists. The resulting amines can be difficult to remove from the product. Phosphonium salt are unstable toward base, degrading to phosphine oxide.

For example, the nucleophilic substitution reaction of an aqueous sodium cyanide solution with an ethereal solution of 1-bromooctane does not readily occur. The 1-bromooctane is poorly soluble in the aqueous cyanide solution, and the sodium cyanide does not dissolve well in the ether. Upon the addition of small amounts of hexadecyltributylphosphonium bromide, a rapid reaction ensues to give nonyl nitrile: