The formation of a toluene-insoluble (TI) fraction and its interaction with fine solids was studied by heating coker gas oil from Athabasca bitumen in a stirred batch reactor at 300-420 degrees C and a total pressure of 13 MPa (from an initial hydrogen pressure of 7 MPa before heating) for 1 h. The Tl solids were filtered from the product oil and analyzed by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectrometry, and elemental analysis. The micrographs indicated that the TI fraction formed a liquid-in-oil emulsion at reactor conditions, consisting of spheres of TI material suspended in gas oil. The spheres were soluble in quinoline, and therefore were not mesophase coke. The TI spheres gave very little physical interaction with 5 mu m kaolin particles, but the addition of particles with non-polar surface groups (carbon black and asphaltene-coated clay) suppressed the formation of spheres. The addition of solid particles did not affect the total yield of TI significantly, based on carbon and sulfur analysis. These results suggested that the liquid TI fraction wetted and spread on non-polar surfaces, but not on polar surfaces.