Coke formation during the refining of heavy oils has attracted extensive attention as a result of the effects on the liquid yield, catalyst deactivation, and operating period. Polycydic aromatic hydrocarbons (PAHs) generally have the strongest tendencies to form coke during the refining processes, which are considered as coke precursors. In this work, a vacuum residue was treated by thermal conversion and deep hydroprocessing. The feedstock and products were characterized by Fourier transform ion cyclotron resonance mass spectrometry. The detailed distributions of aromatic hydrocarbons of the products behaved with clear boundaries, which were described in limit lines. The slopes of the limit lines differed greatly between the two kinds of products, indicating different mechanisms for the growth of PAHs. Thermal conversion and deep hydroprocessing of model compounds were also conducted. Thermal conversion products of phenanthrene and pyrene proved that only condensation reactions occurred at a temperature of 450 degrees C and the aromatic rings were not ruptured. Deep hydroprocessing of pyrene revealed that the aromatic ring structures were cracked and then the smaller aromatic substrates combined together to form highly condensed aromatic rings. As a conclusion, the different mechanisms of coke precursor formation resulted in the different slopes of limit lines for thermal conversion and deep hydroprocessing products.