Disclosed is a hydrothermal treatment process for conversion of a carbon-based energy carrier material. The process comprises a step for sensitizing or activating the carbon based energy carrier material to increase its susceptibility to hydrothermal conversion. As a result of the sensitization step, the hydrothermal conversion step itself may be carried out under relatively mild conditions.The process comprises the steps of sensitizing the carbon-based energy carrier material to increase its susceptibility to hydrothermal conversion; and subjecting the sensitized carbon-based energy carrier material to hydrothermal conversion at a temperature of less than 300 degrees centigrade in a hydrothermal treatment reactor.