The transformation of a synthetic FCC gasoline (2-methylthiophene plus 2,3-dimethylbut-2-ene and orthoxylene dissolved in n-heptane) was studied under mild HDS conditions (total pressure, 20 bar; temperature, 200 degreesC) over a commercial CoMo-alumina catalyst and over the same catalyst modified with potassium. The aim of the study was to identify the factors which can favor HDS and minimize olefin hydrogenation. The modification by potassium of the commercial catalyst made it possible to improve its selectivity in HDS of the synthetic gasoline with respect to the hydrogenation of the olefins. In fact, by decreasing the acidity, the addition of potassium decreases the hydrogenation activity more than the HDS activity. This is considered to be the consequence both of a modification of the electronic properties of the sulfide phase and of the isomerization properties of the support. (C) 2004 Elsevier Inc. All rights reserved.