Oil sands coke is a secondary product of bitumen upgrading via coking. The high sulfur content makes Athabasca fluid coke unusable as a fuel. The removal of sulfur from coke by reaction with hydrogen at high temperatures was investigated. The effects of particle size, temperature and gas flow rates on fixed bed hydrodesulfurization of Syncrude fluid coke were studied. Desulfurisation was found to be affected by the above parameters but showed maximum sensitivity to temperature. The initial global reaction rate decreased about twelve times over 90 minutes. Experimental data showed a good fit with the predictions of the shrinking core model. The process is controlled initially by the gas film and chemical reaction resistances. With time, the diffusion of hydrogen through the increasing ash layer becomes rate controlling. At temperatures ranging from 973 to 998 K the process is chemical reaction controlled while at higher temperatures it is mass transfer controlled.