The ability of solid carbonaceous material to retard the formation of coke during thermal cracking and hydrocracking of heavy hydrocarbons is well known. In this study, we used in-situ microscopy (hot-stage) to obtain additional mechanistic information on whether fine coke and fullerene soot particles retard the growth of mesophase during thermal cracking of Athabasca bitumen, thus reducing the possibility of fouling in preheaters and furnaces. The findings from this study could also have application in other non-catalytic thermal processes such as visbreaking and coking. In the absence of additives in the Athabasca bitumen feed, the formation of mesophase occurred after 61 and 67 min (measured from room temperature) at reaction temperatures of 450 degreesC and 440 degreesC, respectively. The addition of solid coke (ca. 5 wt.%) from a commercial delayed coking operation shortened the mesophase formation time to almost 45-50 min under similar conditions. The coke, having surface area of only 1.65 m(2)/g, resulted in enhanced bitumen fluidity and large-textured mesophase. These observations were rationalized based on the ability of delayed coker coke to release hydrocarbons into the bulk fluid during thermal cracking. Light hydrocarbons released from coke may have changed the solvating power of the liquid phase in bitumen and promoted phase separation, resulting in a shorter induction period. In contrast, adding small amounts of fine fullerene soot (ca. 1 and 5 wt.%) delayed the appearance of mesophase significantly under similar conditions. The ability of fullerene soot to physically absorb the mesophase precursors into its pore structure led to an increase in the apparent viscosity of the bulk phase, which is known to reduce mesophase size and prolong the induction period. Consistent with this, the induction period was prolonged an additional 10 min when the soot surface area was increased from 152 to 208 m(2)/g. The increase in induction period is significant with respect to reaction times and suggests that these fullerene soot materials could be effective in allowing for increased severity and liquid products yield from visbreaking, with less likelihood of fouling in the preheater tubes and furnace walls. (C) 2001 Published by Elsevier Science B.V.