Abstract: Heavy hydrocarbon feedstocks, such as atmospheric and vacuum residua, heavy crude oils and the like, are converted to predominantly liquid hydrocarbon products by contacting said feedstocks in the presence of hydrogen with a regenerable alkali metal carbonate molten medium containing a glass-forming oxide, such as boron oxide, at a temperature in the range of from above about the melting point of said molten medium to about 1000.degree.F. and at elevated pressures. Preferably, the regenerable molten medium comprises an oxide of boron in combination with a mixture of sodium and lithium carbonate or a mixture of sodium carbonate, potassium carbonate and lithium carbonate. The carbonaceous materials (coke) which are formed in the molten medium during the above-described conversion process are gasified by contacting said carbonaceous materials with a gaseous stream containing oxygen, steam, or carbon dioxide at temperatures of from above about the melting point of said medium to about 2000.degree.F.