The ultimate coke yield was determined for agglomerates of Athabasca vacuum residue and coke particles by heating on Curie-point alloy strips in an induction furnace at 503 and 530 degrees C until all toluene-soluble material was converted. Coke yield results from agglomerates were compared to the coke yield results from reacting thin films of vacuum residue. The average coke yield from the agglomerates was 23%, while the coke yield from thin films of 20 mu m thickness was 11%, which supports the role of mass transfer in coke formation reactions. The ultimate coke yield was insensitive to vacuum residue concentration, agglomerate size, and reaction temperature. The temperature profile within agglomerates was measured by implanting a thermocouple, and a simple heat transfer model was used to describe the temperature variation with time. The effective thermal diffusivity of the agglomerates was 0.20 x 10(-6) m(2)/s. Control experiments on reaction in thin liquid films confirmed that heating rates in the range of 14.8-148 K/s had no effect on the ultimate yield of coke yield.