Hydrodesulfurization of dibenzothiophene, dissolved in n-hexadecane, was conducted in the temperature range of 548-593 K at 10 MPa pressure in a conventional packed bed reactor for two different catalyst sizes and on monoliths washcoated with different metal loadings. The monolith catalysts were made by washcoating using NiMo/y-Al203 catalysts prepared by incipient wetness and uniform welladhered washcoats were obtained by calcination after each dipping-air blowing-drying cycle. The Mo03 loading of the powder catalyst was varied from 12.0 to 24.3 wt.%, keeping the Mo03/Ni0 wt. ratio fixed at 6. With an increase in metal loading, the surface area, pore volume and the concentration of weak acidic sites decreased, whereas the metal particle size and the concentration of acidic sites of intermediate strength increased. The highest conversion was obtained with the catalyst containing 18.6 wt.% Mo03 and 3.21 wt.% NiO. The effectiveness factor of the larger size catalyst (average size: 1.5 mm) varied from 0.61 to 0.71 at 593 K. The rate constants (calculated on the basis of metal loading) for the monoliths were in good agreement with those obtained in a packed bed, implying that all the catalyst was utilized in the monoliths. After a 72 h run at 593 K and 10 MPa, the concentration of strong acidic sites increased but there was no change in the activity or product distribution. (C) 2015 Elsevier Ltd. All rights reserved.