Abstract: The invention relates to apparatus for epitaxial processing using metal organic molecular beams. The MOMBE apparatus employs a manifold for supplying metal organic vapor to a reactor which is operated under vacuum. The manifold includes a bubbler in which MO vapor is formed and mixed with a carrier gas. The bubbler provides flexible, three parameter control of the MO reagent permitting use with MO reagents of low vapor pressure. A compensation flow is provided parallelling the reagent flow and employing four valves which are ganged and switched so as to supply the MO carrier gas mixture either to the reactor line or to the vent line and maintain equal flows and pressures during this switching operation. The apparatus is capable of forming very thin reproducible epitaxial layers.