In the gas-phase hydrodechlorination of chlorobenzene to benzene over Ni/SiO2, a pretreatment of the catalyst with HCl or HBr gas induced the growth of highly ordered carbon filaments from the catalyst surface. This filamentous growth was observed to occur at 553 K, i.e., at least 150 K less than that reported previously in the literature for the catalytic formation of graphitic carbon. The hydrogen halide pretreatment also resulted in an appreciable increase in the average nickel particle size and a suppression of hydrodechlorination activity. In the absence of this pretreatment the catalyst exhibited stable hydrodechlorination activity and there was no evidence of any filamentous carbon growth. Analysis of the halide-treated samples revealed the presence of potassium that was not evident in the freshly activated catalyst but was introduced as a result of a corrosive attack of the hydrogen halide gas on the microreactor unit. The source of our low-temperature ordered carbon growth must be electronic in nature and is attributed to an intimate combination of halogen/alkali metal on the catalyst surface that facilitates a restructuring of the active sites where a destructive chemisorption of chlorobenzene precedes the dissolution of carbon and precipitation in an ordered fashion.