The gas phase hydrodechlorination of single component and mixtures of o-, m- and p-chlorophenol was studied over the temperature range 423 K less than or equal to T less than or equal to 573 K using a 1.5% (w/w) Ni/SiO2 catalyst. The variation of catalyst activity with time-on-stream for each isomer is illustrated and the role of reaction temperature and thermodynamic limitations are addressed. The catalyst exhibits both short term and irreversible long term deactivation which is accounted for in terms of competitive adsorption and electronic effects. The hydrogen treatment of both phenol and chlorobenzene under the same reaction conditions are also considered for comparative purposes. The presence of the hydroxyl function enhances the rate of hydrodechlorination via an inductive effect. The relationship between rate and isomer structure is discussed on the basis of reactant adsorption where steric hindrance, in the case of the ortho-form, appreciably restricts dechlorination to such an extent that o-chlorophenol remains unreacted in equimolar o-/p- and o-/m-chlorophenol mixtures. Catalytic hydrodechlorination is viewed as a non-destructive low energy methodology for handling concentrated chlorine gas streams and relationships that describe the dependence of dechlorination rate on chlorine concentration ape provided and can be used to evaluate the productive lifetime of the catalyst.