A system for supplying processing fluid to a substrate processing apparatus having walls, the inner surfaces of which define a processing chamber in which a substrate supporting susceptor is located. The system consists of a number of fluid storages, each which stores a separate processing fluid, at least two fluid conduits along which processing fluid flows from the fluid storages to the processing apparatus and a fluid inlet which connects the fluid conduits to the processing chamber. The inlet has a separate fluid passage, corresponding to each of the fluid conduits, formed along it. Each fluid passage opens at or near an inner surface of a wall to define a fluid mixing zone, so that fluid moving along one fluid passage is prevented from mixing with fluid moving along any other passage until reaching the mixing zone.