The gas-phase hydrodechlorination of pentachlorophenol (PCP) over nickel/silica and nickel/Y zeolite catalysts at 573 K has been studied. Each catalyst was 100% selective in cleaving the C-Cl bonds, leaving the hydroxyl substituent and benzene ring intact. The variation of catalytic activity and selectivity (in terms of partial and full dechlorination) with time-on-stream is illustrated and catalyst deactivation is addressed. Dechlorination efficiency is quantified in terms of dechlorination rate constants, phenol selectivity/yield and the ultimate partitioning of chlorine in the parent organic and product inorganic host. Increasing the nickel loading on silica was found to raise the overall level of dechlorination while the use of a zeolite support introduced spatial constraints that severely limited the extent of dechlorination. Product composition was largely determined by steric effects where resonance stabilisation had little effect. The reaction pathways, with associated pseudo-first-order rate constants, are also presented.