A method of upgrading a hydrocarbon feedstock, the method comprising the steps of: preheating the hydrocarbon feedstock to a temperature of up to 250°C; preheating a hydrogen donating composition to a temperature of greater than 50°C and up to 500°C and maintaining said temperature for a period of at least 10 minutes; preheating the water to a temperature of up to 650°C; and mixing the preheated hydrocarbon feedstock stream, the hydrogen donating composition, and the water stream to produce the mixed stream; supplying a mixed stream comprising the hydrocarbon feedstock, water and a pre-heated hydrogen donating composition to a hydrothermal reactor, wherein the mixed stream is maintained at a pressure greater than the critical pressure of water and a temperature greater than the critical temperature of water; reacting the mixed stream in the hydrothermal reactor in the absence of catalyst; reacting the mixed stream in the hydrothermal reactor to convert the mixed stream into a modified stream comprising upgraded hydrocarbons; separating the modified stream into a gas stream and a liquid stream; and separating the liquid stream into a water stream and an upgraded hydrocarbon product stream; wherein the hydrogen donating composition is a bottoms streams from a process selected from the group consisting of hydrocracking, coking, visbreaking, hydrotreating, or catalytic cracking and is selected from the group consisting of tetralin, alkylated tetralin, extracts of liquefied coal, petroleum refinery distillates, cracked products from a petroleum refinery product stream, residue from a petroleum refinery, and combinations of the same and the weight ratio of the hydrogen donating composition to the hydrocarbon feedstock is between 0.005:1 and 0.1:1. preheating the hydrocarbon feedstock to a temperature of up to 250°C; preheating a hydrogen donating composition to a temperature of greater than 50°C and up to 500°C and maintaining said temperature for a period of at least 10 minutes; preheating the water to a temperature of up to 650°C; and mixing the preheated hydrocarbon feedstock stream, the hydrogen donating composition, and the water stream to produce the mixed stream; supplying a mixed stream comprising the hydrocarbon feedstock, water and a pre-heated hydrogen donating composition to a hydrothermal reactor, wherein the mixed stream is maintained at a pressure greater than the critical pressure of water and a temperature greater than the critical temperature of water; reacting the mixed stream in the hydrothermal reactor in the absence of catalyst; reacting the mixed stream in the hydrothermal reactor to convert the mixed stream into a modified stream comprising upgraded hydrocarbons; separating the modified stream into a gas stream and a liquid stream; and separating the liquid stream into a water stream and an upgraded hydrocarbon product stream; wherein the hydrogen donating composition is a bottoms streams from a process selected from the group consisting of hydrocracking, coking, visbreaking, hydrotreating, or catalytic cracking and is selected from the group consisting of tetralin, alkylated tetralin, extracts of liquefied coal, petroleum refinery distillates, cracked products from a petroleum refinery product stream, residue from a petroleum refinery, and combinations of the same and the weight ratio of the hydrogen donating composition to the hydrocarbon feedstock is between 0.005:1 and 0.1:1.