The coking propensity (mesophase appearance time and size) of a fraction of mineral-matter-free Athabasca bitumen vacuum bottoms was investigated using hot-stage (high-temperature and high-pressure) microscopy, The following variables were studied: (1) gas atmosphere (hydrogen and nitrogen); (2) different clay minerals and concentrations (kaolinite at 2 wt % and 5 wt %, illite at 5 wt %, and montmorillonite at 5 wt %). The results of repeated experiments indicated that the mesophase induction period was relatively insensitive to the gas atmosphere. However, the size of the mesophase measured over the same time period was larger and the growth was uninhibited under hydrogen compared with nitrogen. It was also observed that the use of different clays at different concentrations did not significantly affect the mesophase induction period. Kaolinite, at the same concentration, had a significant effect in reducing the size of the mesophase compared with illite and montmorillonite. The effect was more pronounced at higher concentrations of kaolinite. The formation of a small-size mesophase in the case of kaolinite can be explained in terms of the increasing apparent viscosity of the isotropic matrix. The possible mechanism of the clay-mesophase interaction is envisaged as follows: Clays stabilize the layer structure of mesophase, thus inhibiting the shear that is the characteristic form of mesophase deformation. The presence of kaolinite clay prevents the molecules in the pitch matrix from acting as lubricants and restricts the coalescence of the mesophase. Differences in the physical properties of the clays (shape, size, surface charge, and hydrophilicity) may explain the differences in the observed interactions between these clays and the mesophase. The present findings may have significant implications in delayed coking operations and in the prevention of foulant adhesion to the internal wall of furnaces and superheaters.