Hydrodesulfurization of Athabasca (Syncrude) fluid coke was studied for particle sizes of -74 + 53, -53 + 44 and -44 mu m using a quartz reactor. Five flow rates of hydrogen from 1.2 x 10(-6) to 2.5 x 10(-6) m(3)/s were investigated with from 0.45 to 1.0 g of coke. Gas production - time profiles for H2S and CH4 were obtained at temperatures from 973 to 1073 K for each particle size range. Desulfurization rates were functions of particle size and temperature. Results agree with predictions of the shrinking core model, the rate being controlled initially by the gas film and chemical reaction resistances followed by control due to diffusion of hydrogen through the increasing ash layer. Below 998 K, the apparent activation energy was determined to be 293 kJ/mol . K, while at temperatures between 998 and 1073 K it was 29 kJ/mol . K.