A cooling system for molding fixtures, and particularly for foundry molds, comprising: water treatment means; compressed air generation means connected to an air line; means for pressurizing the treated water, which is connected to the water treatment means and is connected to a water line; a device for mixing air with the treated water and for the controlled ejection of atomized water under pressure toward a cooling circuit of a molding fixture, which is connected to the compressed air generation means and to the water pressurization means. The mixing and ejection device comprises: a nozzle, which is connected to the water line and has a water exit hole, and a three-way connector connected to the nozzle and to the air line. The operating method comprises: the water pressurization means pressurizes treated water into the water line, so that treated water under pressure is caused to pass through the nozzle that yields, through the water exit hole, a predefined flow-rate of water under pressure; the compressed air means pressurizes air into the air line; treated water under pressure from the nozzle and compressed air from the compressed air generation means enter in the three-way connector and are mixed inside the three-way connector, so that the mixing and ejection device ejects a controlled ejection of atomized water under pressure toward a cooling circuit of a molding fixture; wherein compressed air exiting from the compressed air generation means is at a first preset pressure value; wherein treated water under pressure exiting from the water pressurization means is at a second preset pressure value; wherein the second preset pressure value of the treated water under pressure is at least one bar higher than the first preset pressure value of the compressed air. In particular, such a system is applied on molds for the production of castings of light alloys, permanent-mold castings, low-pressure or high-pressure die castings, but it should be understood that it can be applied to molding fixtures in general. The aim of the present invention is to provide a cooling system for molding fixtures in foundries, which is capable of overcoming the drawbacks of conventional cooling circuits and cooling systems. Within this aim, an object of the invention is to provide a cooling system that is capable of combining the advantages of air systems with the effectiveness of water circuits, and to provide a cooling system that can also be fitted on existing molding fixtures. Another object of the invention is to provide a cooling system that does not produce thermal shocks in the mold with which it is associated, and which is immune to leaks owing to problems of excess pressure.