Co—Mo, Fe and Mo catalysts were prepared on alumina and carbon supports. The reactivity of sulphided catalysts for thiophene hydrodesulphurization and the propensity for coke deposition of nonsulphided catalysts during propylene cracking and anthracene carbonization were measured. The hydrodesulphurization activity of Co—Mo and Fe catalysts increased in the order ¿-Al2O3 <C-black composite <active carbon. For a given metal, considerable differences in activity were found both between the carbons and alumina and between the carbons themselves. Possible reasons for the different activities include the influence of the supports on metal dispersion and the presence of impurities in the carbons. For alumina supported catalysts there is a correspondence between hydrodesulphurization activity and coking propensity. On carbon supported catalysts, the rates of carbon deposition appear insensitive to the nature of the metal component(s) whereas hydrodesulphurization activity is not. Thus it is possible to enhance the latter without attendant increases in the susceptibility to coke-forming reactions. Increasing Mo loading from 0–12 wt% was found to substantially increase the rate of carbon deposition on alumina, whereas the effect on a carbon support was comparatively small. This behaviour appears to relate to the inherent support acidity and its modification on metal addition.