Abstract: An apparatus for guiding gas through a tube reactor in low pressure chemical vapor deposition processes. The apparatus includes the tube reactor, a substrate mounting element in the interior of the tube reactor, to receive substrates in a generally parallel position with respect to each other along the substrate mounting element, a cup-shaped inner tube surrounding the substrate mounting element, a gas feed arranged within the inner tube, a gas discharge opening arranged in the inner tube at a point opposite to the gas feed, and an intermediate zone between the tube reactor and the inner tube. Preferably, the inner tube is eccentrically disposed within the tube reactor, to increase the size of the intermediate zone in the area of the gas discharge opening, thus facilitating passage of exhaust gas out of the inner tube. Typically, the substrate mounting can be rotated.