The mechanism of formation of carbonaceous deposits from hydrocracked Athabaca bitumen is shown to result in both isotropic and anisotropic solids. The formation of mesophase spheres present as a separate liquid phase has been observed in the laboratory High severity results in the formation of an increased amount of anisotropic solid which is observed in both laboratory and commercial samples. The formation of these deposits in the downstream separators of a commercial plant has occurred and is a hindrance to reliable operation. A strategy to reduce the formation of these deposits in a commercial plant was evaluated in a series of laboratory experiments. The use of process-derived gas oils can be used as quench oil to lower the temperature of the commercial separators. There was no increase in the rate of toluene insoluble formation as a result of the use of quench oils, which allowed the full advantage of the temperature decrease to be achieved. A qualitative comparison of the build-up of deposits in the commercial separators is provided from separator vessel monitoring.