Hydrocracking of n-decane was carried out on a sulfided NiW/silica-alumina catalyst in a fixed-bed reactor. in the presence of hydrogen sulfide and ammonia. The effect of the reaction temperature and of the partial pressures of n-decane, hydrogen, hydrogen sulfide and ammonia was determined. The transformation of n-decane occurred through two reactions: a bifunctional transformation and a "direct cracking" reaction. Whatever the operating conditions, the main reaction observed was the bifunctional transformation of n-decane into monobranched isomers. At low reaction temperature or in the presence of ammonia. the bifunctional transformation of the monobranched isomers into multibranched isomers and into cracking products was very slow. This can be related to the weak acidity of the silica-alumina support which was moreover strongly poisoned by ammonia. Consequently, the acidic step of n-decane, hydrocracking was clearly rate-determining. which was in agreement with the kinetic reaction orders measured. The "direct cracking" reaction always existed. whatever the operating conditions, and yielded linear C-2-C-8 alkanes. The kinetic orders measured for this reaction confirmed that it occurred on exposed W atoms in the sulfide phase of the catalyst. (C) 2004 Elsevier B.V.. All rights reserved.