The hydrodehalogenation (HDH) of fluorobenzene (FB), chlorobenzene (CB), bromobenzene (BB), iodobenzene (IB), 1,3-dichlorobenzene (1,3-DCB), 1,3-dibromobenzene (1,3-DBB) and 1,3-bromochlorobenzene (1,3-BCB) has been studied over Ni/SiO2 at 573 K. In the case of the mono-haloarenes, HDH activity decreased in the order FB > CB > BB > IB, an effect that is discussed in terms of halogen inductive effects. The conversion of the di-substituted benzenes (at a common inlet X/Ni molar ratio) was significantly lower (by a factor of up to 2) than that recorded for the mono-haloarenes, i.e. the presence of a second electron withdrawing halogen substituent lowered the overall level of HDH. A decline in HDH activity with time-on-stream was observed in every instance. Catalyst activation via direct temperature-programmed reduction (TPR) has been monitored with on-line thermogravimetric/mass spectrometric/thermal conductivity analyses. The incorporation of a calcination step prior to reduction resulted in a marked decline in HDH activity. The pre- and post-reaction catalyst samples have been characterized by BET area measurements, X-ray diffractograms (XRD) and H-2 chemisorption/temperature-programmed desorption. TPR analyses of the post catalysis (passivated) samples were also conducted and compared with the response recorded for the unused catalysts. The loss of activity is linked to halogen/Ni interactions resulting in a suppression of H-2 uptake with possible Ni sintering and a disruption of the H-2/catalyst interactions. In the hydrotreatment of 1,3-BCB, the level of debromination exceeded that of dechlorination, a result that we attribute to a surface hydrochlorination and Cl exchange with the Br substituent on the aromatic ring. This effect extends to reactions over bulk NiO and NiO + SiO2 mixtures. (C) 2004 Elsevier B.V. All rights reserved.