Supercritical Water Hydrothermal Synthesis (scWHS) is a relatively simple and environmentally friendly process for the production of potentially valuable metal oxide nanoparticles. Previous problems with blockages forming in the original T piece reactor were overcome by redesigning the reactor using image analysis and computational fluid dynamics. An optimised reactor, termed the Nozzle Reactor, has been developed which can be run continuously and is able to produce a range of different metal particles including titania, ceria, zirconia, copper oxide, YAG, hematite, magnetite and silver. The reactor also shows a dramatic improvement in process reproducibility (± 5m2/g for BET surface area) and in control of particle size. Preliminary evidence suggests that the reactor could eventually lead to the ability to good control of particle properties, such as size, composition and shape, through the manipulation of process variables.