Catalysts used in the hydroprocessing of heavy feedstocks deactivate due to coke deposition. Although the mechanism for coke formation is not fully understood. condensation reactions of polynuclear aromatic compounds present in the feeds are a key aspect. The effect of coke deposition on catalyst performance is typically modelled using accelerated aging agents comprising model coke precursors. Mixtures employed in such studies include polynuclear aromatic compounds such as anthracene as the coke precursor, and long chain alkanes such as hexadecane as a diluent. We show in this paper that binary and pseudo binary mixtures of polynuclear aromatic compounds and n-alkanes present Type 11, Type IV, or Type III phase behaviour according to the van Konynenburg and Scott phase projection nomenclature. Incubation periods and the apparent autocatalytic effects associated with coke deposition in such systems are explained through a combination of high temperature phase equilibrium experiments and computations with the model systems n-alkane + anthracene + hydrogen, n-alkane + pyrene + hydrogen. and n-alkane + dibenzo[a,k]chrysene + hydrogen.