The conditions of third-phase formation in the system made up of aqueous solution of citric acid-solution of trialkylamine in a binary diluent were studied. As binary diluents mixtures of octanol-l or chloroform with n-heptane were used. The experiments were carried out in water-free systems, at 25 degrees C under the limiting condition of amine fully loaded with acid, thus simulating extraction from highly concentrated aqueous phase. The data could be presented as pseudo-ternary equilibria and were correlated using the extended method, devised by Rod (Chem. Engng J. 1976, pp. 105-110).