The hydrocracking of n-decane was carried out on a sulfided NiW/silica-alumina catalyst and for comparison on a sulfided NiW/USHY zeolite catalyst (fixed-bed reactor, 380 degreesC, 6 MPa total pressure, presence of sulfur and nitrogen-containing compounds in the feed). As could be expected from the weak acidity of the silica-alumina support, hence from the high ratio between the hydrogenating and the acid functions, at high n-decane conversion NiW/silica-alumina was found more selective than NiW/zeolite for the formation of isomerization products. The difference in selectivity between the two catalysts was even more significant at low n-decane conversion, due to a direct cracking of n-decane which occurred on NiW/silica-alumina in addition to the classical bifunctional process. This reaction was shown to take place on the sulfided NiW phase, possibly through abstraction of a proton of the molecule by a basic surface sulfur atom. (C) 2003 Elsevier Inc. All rights reserved.