Abstract: An MOCVD deposition technique wherein the pressure of reagent vapors within the reaction vessel (1) is maintained at a reduced pressure in the range 10.sup.-2 to .about.10 millibars and contained mounted substrates (15) are heated non-inductively e.g. by an electric resistance circumferential furnace (5). In the above pressure range, high diffusivity of the reagent vapors ensures exposure of the substrates (15) to a uniform reagent mixture. A large number of substrates (15) may be processed simultaneously. As heating is non-inductive, an inert mounting (17) can be utilized avoiding the introduction of contaminants (e.g. carbon) into the deposited film.