Semi-commanded valve system applied to an alternative-type compressor with at least a cylinder (1), at least a piston (2), at least a compression chamber (3), and at least one valve (71;81) acting in their respective orifice (7;8). Said at least one valve (71;81) is prestressed in a first operation state and is able to act as a check valve. Furthermore, said at least one valve (71;81) has at least a ferrous portion and can be selectively actuated by a magnetic field generating element (72;82). Method of modulating the capacity of a compressor provided with such a semi-commanded valve system.