Hydrodechlorination of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), pentachlorophenol (PCP), and p-chlorobenzoic acid with hydrogen gas over 5% Pd/C or Raney-Ni was studied in different liquid phase systems: (1) ethanol or ethanol-aqueous KOH; (2) in the multiphase system, composed by an organic phase and aqueous KOH and containing a quaternary ammonium salt (Aliquat 336). Under mild conditions (50degreesC and atmospheric pressure of hydrogen) quantitative hydrodechlorination of 2,4,5-T is achieved in 1-4 h over Raney-Ni or Pd/C in the multiphase system or in the ethanol-aqueous KOH solution. PCP reacts selectively giving the dechlorinated phenolate in several hours in the system made of aqueous KOH (with Raney-Ni or Pd/C) or in several minutes in the presence of a small amount of Aliquat 336 solution in isooctane (1-4 molar% of Aliquat 336), where Aliquat 336 promotes the phase-transfer of polychlorinated anion. The reaction rates versus conditions, process localization, and the role of the ionic liquid environment are discussed. (C) 2004 Elsevier B.V. All rights reserved.