A system (8; 9; 50) is described of walls for catalytic beds of synthesis reactors (1), in which there is a wall (14) in direct contact with a catalytic bed (7) for containing it, said wall (14) having a plurality of portions (17) permeable to the gases and a plurality of portions (19; 54; 55) impermeable to the gases, said portions (17) permeable to the gases being equipped with slits (18; 52, 53; 60; 70) of a size such as to allow the free passage of the synthesis gases through them but not the passage of the catalyst, in which the slits are obtained with milling, water cutting or electro-erosion processing.