Pure and mixed delta/alpha-aluminas were prepared and modified (treated with HCl gas and aq HNO3) in order to be studied as supports of nickel catalysts for the hydrodechlorination reaction of 1,2,4-trichlorobenzene in the gas phase. All the samples were structurally characterized using XRD, BET, TPR, SEM and TPD techniques. The catalytic activities for the hydrodechlorination of 1,2,4 trichlorobenzene are higher for the catalysts containing the modified supports (especially those treated with HNO3) and can be explained by the reduction degree (TPR), the structure (BET area, TPD and SEM) and the number and type of catalytic active sites (H-2 TPD). The activity can be related to the hydrogen available at the reaction temperature (423-523 K) while the selectivity towards benzene can be explained in terms of the hydrogen desorbed at lower temperatures (350-500 K) which competes with the aromatic compounds to be adsorbed on the surface of the catalyst. The selectivity towards benzene is directly related to the activity for each catalyst. Nevertheless, the most selective catalyst towards benzene is the one supported on a-alumina (73% of selectivity at 523 K) which is not the most active. This may be correlated to the hydrogen desorbed at low temperatures.