The deposition of coke onto catalysts during the conversion of heavy vacuum gas oil has been investigated in a trickle-bed reactor at elevated temperatures (ca. 450-degrees-C) and moderate hydrogen pressures (30 bar). It is argued that coke is formed via two parallel routes, viz.,(i) thermal condensation reactions of aromatic moieties and (ii) catalytic dehydrogenation reactions. The amount of coke deposited onto the catalyst shows a distinct maximum as a function of both temperature and hydrogen/oil ratio. These maxima are induced by the vapor-liquid equilibria in the reactor. A qualitative description involving the concentration of heavy coke precursors present in the oil phase is given.