The present invention relates to an apparatus, method, a reaction chamber and a use of a reaction chamber for processing a surface of a substrate by subjecting the surface of a substrate to successive surface reactions of at least a first precursor and a second precursor. The apparatus comprises a vacuum chamber; a detachable reaction chamber arranged to be installed inside the vacuum chamber, and inside which the substrate is positioned during processing and a precursor system for supplying the at least first and second precursors into the action chamber and for discharging the at least first and second precursors from the reaction chamber. According to the present invention the reaction chamber is provided as a gastight vessel.