The performance of sulfided CoMo hydrotreating catalysts supported on MCM-41 has been evaluated using hydrodesulfurization of 4-ethyl, 6-methyl- dibenzothiophene (4E, 6M-DBT) model compound in n-hexadecane and of 4-methyl, 6-methyl-dibenzothiophene (4,6- DMDBT) in a commercial feed as a test reaction. Although the performance of CoMoS/MCM-41 is high for the model feed reaction as compared to commercial CoMoS/alumina, for the commercial feed reaction the performance of the commercial catalyst is much better. Model feed reaction with the addition of carbazole shows that, although this nitrogen-containing compound has a strong inhibiting effect, the lower catalytic performance of CoMoS/MCM-41 in the commercial feed test is probably not solely due to the presence of nitrogen.TEM investigations show that the decrease in activity of the CoMoS/MCM-41 in the nitrogen-containing and commercial feed reactions is not due to sintering of the sulfidic phase. However, it appears that the MCM-41 collapses during the catalytic reactions, and this effect is most pronounced for the commercial feed test. The degradation of the MCM-41 support material does not affect the dispersion of the CoMoS, but induces increased coking.