Abstract: An instantaneous steam generator is disclosed which has a plurality of heated channels having open tops. Each channel has a base having a steam generation surface with substantially parallel, sharp transverse ridges. The steam generation surface is heated to a temperature substantially above that at which an insulating vapor barrier forms between the surface and a water layer thereabove. The water flow is directed transversely over the ridges so that the vapor layer is separated from the water layer above and so that the water is instantaneously converted into steam rather than riding on an insulating layer of water vapor. The channels can be enclosed in a box having openings. The box is pressurized by a blower drawing relatively cool, ambient air so that relatively wet steam is distributed throughout the entire interior volume of a large oven. Control apparatus for heating the channels and controlling the water flow thereto are described.