the invention relates to an apparatus for growing thin films onto the surface of a substrate by exposing the substrate to alternately repeated surface reactions of vapor - phase reactants . the apparatus comprises at least one process chamber having a tightly sealable structure , at least one reaction chamber having a structure suitable for adapting into the interior of said process chamber and comprising a reaction space of which at least a portion is movable , infeed means connectable to said reaction space for feeding said reactants into said reaction space , and outfeed means connectable to said reaction space for discharging excess reactants and reaction gases from said reaction space , and at least one substrate adapted into said reaction space . at least one loading chamber is arranged to cooperate with said process chamber so as to permit said reaction space or a portion thereof to be moved into said process chamber and away from said process chamber and , further , the operating pressure of the loading chamber is arranged to be controllable independently from said pressure chamber .