The activity-selectivity-stability pattern of a 19 wt% Ni/MgO catalyst in the prereforming of n-hexane (T 450 degrees C; P, 10-15 bar; S/C, 1.5-3.5), in the absence and the presence of H-2 (H-2/C, 1), is thoroughly addressed. Coking and, to a much lesser extent, sintering phenomena affect the catalyst stability with a 1(st)-order dependence of deactivation rate on activity. Regardless of reaction conditions, in the absence of hydrogen a general selectivity-conversion pattern signals that CO is a primary reaction product from the steam reforming reaction, whereas the prevalence of a hydrogenolysis path enhances reaction rate, methane selectivity, and catalyst stability in the presence of hydrogen, markedly hindering the coking process. Driven by different ensembles of active sites, steam reforming, CO methanation, water-gas-shift, hydrogenolysis, and coking functionalities control the behavior pattern of the Ni/MgO catalyst in the pre-reforming process. (c) 2006 American Institute of Chemical Engineers AIChE J, 52: 2823-2831, 2006.