Diesel fuels containing 50 and 15 ppm sulfur were produced in a pilot plant by upgrading light catalytic cracking gas oil (LCO) during 10 weeks of continuous operation. At the end of the run, the catalysts were characterized before and after soluble coke extraction by CH2Cl2. The cokes were characterized by C-13 NMR, TPO, GC-MS, and elemental analysis. Catalyst surfaces were characterized by XPS, CO adsorption, pyridine adsorption, and chemical reactions. The results indicate important differences in the amount and composition of soluble coke recovered from the two deactivated catalysts. In the two cases, the soluble coke affected the accessibility of catalytic active sites in different ways. Catalyst deactivation was higher, and the rate of ring opening was lower, under the more severe hydrotreatment conditions needed to produce fuel with 15 ppm sulfur, compared to the conditions required for production of fuel with 50 ppm sulfur.