The yield of coke as a function of time was measured for vacuum residue from Athabasca in a batch reactor at 430 degrees C, The presence of fine solids in the residue inhibited coke formation, delaying the onset of coking and reducing the yield of coke by as much as 7-9 yield % after 20-30 min. Similar trends were observes in coke formation from an asphaltene fraction of Athabasca bitumen. Experiments with an asphaltene fraction showed that coke yields at short reaction times were very sensitive to subtle changes in the reaction mixture. These results suggest that fine solid’s may help to disperse a coke-precursor phase at reactor conditions, preventing coalescence and thereby altering the rate of formation of solid coke.