The toluene-insoluble solids obtained from laboratory hydrocracking experiments were analyzed to investigate the types of compounds involved in the formation of these solids. Thermogravimetric analysis of the toluene-insoluble solids showed that the weight loss was directly dependent on the hydrocracking reaction severity under which the toluene-insolubles formed, The toluene-insoluble solids were also pyrolysed and the products were analyzed using gas chromatography and mass spectrometry (GC-MS). The toluene-insoluble solids obtained by hydrocracking at low reaction severities released alkanes and alkenes in addition to aromatic compounds, while the toluene-insoluble solids obtained from high severity hydrocracking released predominantly aromatic compounds. The results suggested that the predominant type of toluene-insoluble solids formed depended upon the relative amounts of compounds containing aromatic rings with and without side chains.