The influence of oil fractions' compositions on the conversion of sulfurous components was investigated in a trickle-bed reactor in laboratory scale. A commercially available NiMo/gamma-Al2O3 catalyst was used throughout the investigations. Experimental results including sulfur conversion of different oil fractions and residence time distributions under reacting conditions are presented. The hydrogen consumption is ascribed to the conversion of sulfur and of nitrogen, to the hydrogenation of aromatics and to hydrocracking based on a simulation applying ASPEN Plus(R). Various configurations of the desulfurization process are evaluated but no advantage is found by separate treatment of individual oil fractions. In addition, experiments were carried out to determine liquid distribution and wetting efficiency in a catalyst bed by using magnetic resonance imaging (MRI) technique. (C) 2003 Elsevier Science B.V. All rights reserved.