A refrigerant compressor is disclosed, having a piston (2) which is movable back and forth in a cylinder (3), the cylinder (3) having at its end wall (5) a discharge opening (6') closable by a discharge valve (7'), into which opening a projection (16') on the corresponding end face of the piston (2) projects when the piston is located in the region of its end position. It is desirable to increase the efficiency of such a compressor. For that purpose, the projection (16') is of asymmetrical construction in cross section.