The reactivity of different organic compounds ( naphthalene, alkylnaphthalenes, decalins, tetralin and their mixtures) and the roles of the support acidity and of the noble metals were studied in Pd/Pt on MCM-41 ( with different Si/Al ratios) catalysts. The catalytic tests showed that the hydrogenolysis/ring-opening reactions mainly occurred on saturated compounds, while cracking took places mainly on unsaturated compounds. The presence of alkyl side chains on the polyaromatic ring inhibited the hydrodearomatization activity, proportionally to their length and number. Mixtures of naphthalene and methylnaphthalenes gave rise to a competition between the substrates, with a decrease in both hydrogenation and hydrogenolysis/ring-opening activities. Increasing support acidity favoured hydrogenolysis/ring-opening and cracking reactions, smoothing the hydrogenation activity. Noble metals are shown to be necessary not only in hydrogenation, but also in hydrogenolysis/ring-opening reactions.