An apparatus for processing wafer-shaped articles comprises a closed process chamber providing a gas-tight enclosure. A rotary chuck is located within the closed process chamber, and is adapted to hold a wafer shaped article thereon. A lid is secured to an upper part of the closed process chamber. The lid comprises an annular chamber, gas inlets communicating with the annular chamber and opening on a surface of the lid facing outwardly of the closed process chamber, and gas outlets communicating with the annular chamber and opening on a surface of the lid facing inwardly of the closed process chamber.