A hydrothermal conversion process includes a mixing step wherein an aqueous slurry of a solid feedstock material with a steam stream to produce a reaction mixture having a temperature of at least 160° C. and which is at a pressure sufficient to keep water as a subcooled liquid. The process is fast and effective, requires only simple equipment and is highly energy-efficient. The process is also readily scalable, can be operated continuously or semi-continuously and can be tailored to produce carbonized solids or liquefaction products, all of which typically have increased economic value compared with the starting materials.