Abstract: A process for producing lower average molecular weight products from heavy liquid hydrocarbons is provided comprising contacting the feed material in the presence of a co-catalyst system comprising a molten salt of sodium tetrachloroaluminate (NaAlCl.sub.4) and hydrogen tetrachloroaluminate (HAlCl.sub.4) at a pressure of from about 0.8 to about 140 atm (81 to about 14185 kPa) and a temperature of from about 200.degree. C. to about 550.degree. C.According to the present invention use of the HAlCl.sub.4 as a co-catalyst with NaAlCl.sub.4 results in increased yields of lower average molecular weight products and improved levels of denitrogenation and desulfurization. The elements of the co-catalyst system may be prepared separately and mixed.