In the search for active catalysts for the conversion of refractory sulfur compounds in diesel fuel, the activity, the role of the support, and the nature of the active sites on Pt/ASA catalysts in deep HDS reactions were studied and compared to Pt/gamma-Al2O3 and Pt/XVUSY (stabilised Y zeolite). Pt/ASA appears to be much more active than Pt/gamma-Al2O3 but is initially less active than Pt/XVUSY. The latter however, showed strong deactivation after short reaction times. It is concluded that an appropriate tuning of the support acidity is crucial for this reaction. In contrast to the activity, the H2S sensitivity of the tested Pt based catalysts is hardly influenced by acidity of the support. Adsorption of H2S on these sulfur vacancies leads to strong competitive adsorption with the reacting sulfur compound. It is proposed that the stabilisation of small platinum clusters in the presence of H2S is an important effect of acidic supports. In addition, the strength and nature of the acidic sites on the support may affect the Pt-S bond strength of the active sites on small platinum particles. It is concluded that no sulfur-free platinum metal sites are present under the applied reaction conditions. It is therefore proposed that the conversion of 4-E,6-MDBT over Pt/ASA proceeds over sulfur vacancies on small platinum particles. The creation of sulfur vacancies on these small platinum particles may be related to their electron-deficient character on acidic supports.