Hydrodehalogenation of iodo-, bromo-, and chlorobenzene on 5% Pd/C is studied in an ethanolic base solution and in the aqueous base-isooctane system at 50 degreesC and atmospheric pressure of hydrogen. Rates and selectivity of competitive and separate reactions of different halides and the effects due to the quaternary ammonium salt, Aliquat 336, are discussed. The reactivity of halides follows the order PhCl approximate to PhBr > PhI in the biphasic system. Aliquat 336 promotes hydrodechlorination, but inhibits hydrodebromination and hydrodeiodination. The rate inhibition in the reaction of PhI is connected to formation of the iodide-substituted ammonium salt, which holds inside Pd/C and reinforces poisoning by I-, while the presence of the excess of Aliquat 336 prevents inhibition. In competitive experiments, the phenyl halides react selectively one after another according to the order of their adsorption on Pd: PhI, PhBr, PhCl, where hydrodechlorination and hydrodebromination are slowed down by the iodide anion formed in hydrodeiodination. (C) 2004 Elsevier Inc. All rights reserved.