CoMo on Al2O3 catalyst prepared by spray pyrolysis method was found in the form of spheres of 0.5-1.21 mum, which consisted of tiny primary particles of ca. 10-20 nm diameter. The materials shows comparable activity to those of commercial catalysts in HDS of straight run gas oil, in particular, refractory 4,6-dimethyldibenzothiophene (4,6-DMDBT). Weaker interactions between CoMo and Al2O3 are suggested by temperature-programmed reduction (TPR), Raman spectroscopy, to give more active species than those over the impregnated catalysts. This accounts for its comparable activity in spite of its smaller surface area. (C) 2003 Elsevier B.V. All rights reserved.