Abstract: A reactor for coating flat substrates and particularly wafers is described which has a reaction vessel into which reaction gases can be introduced, and a substrate holder unit in which substrates are held in a holder such that the main surface of the substrates to be coated is oriented downward during the deposition operation and is aligned essentially in parallel to the flow direction of the reaction gases. The invention is characterized in that at least two spaces for substrates are provided on the substrate holder unit and in that the holder or holders is/are constructed in the manner of a template which has openings for the surfaces of the substrates to be coated.