The influence of the Si/Al ratio of the support of(Co)Mo/saponite catalysts on the performance in the hydrodesulfurisation of thiophene and hydrocracking of n-decane has been studied. The initial thiophene HDS activity of the sulfided Co catalysts increased with increasing support acidity, while with the CoMo catalysts the opposite effect was observed. The large drop in activity at lower Si/Al ratios is ascribed to the occurrence of two separate cobalt and molybdenum sulfide phases instead of the "Co-Mo-S" phase. In the hydrocracking of n-decane both the Co and the CoMo catalysts displayed an almost linear relationship between the first-order rate constant and the number of Bronsted acid sites. As with zeolite catalysts the Si/Al ratio was also found to affect the product selectivity. At low Si/Al ratios the relative amount of secondary cracking products increased due to a changing balance between the number of cracking and hydrogenation sites. Remarkably it was found that in the presence of H2S between 423 and 573 K the n-decane conversion passes through a maximum. The observed relation between this "low-temperature" activity and the number of Bronsted acid sites seem to indicate the involvement of both H2S and the acid sites on the support.