Abstract: A coolant accumulator for use with the cooling system of an internal combustion engine, having a cooling radiator with a filler opening in it, provides positive anti-aeration of the engine cooling system and positive recovery of all coolant. This is accomplished by mounting a dual-compartment recovery tank above the filler opening of the radiator to permit free flow of fluid between the filler opening of the radiator and the bottom of a high pressure compartment of the tank. A pressure-actuated one-way valve is located in a passage interconnecting the top of the high pressure compartment with a second compartment. The valve is opened when the pressure in the radiator (and consequently, the high pressure tank) is a predetermined amount above ambient atmospheric pressure to permit fluid to flow from the high pressure compartment in the second compartment. The second compartment is vented to the atmosphere and has a filler opening in it to permit the addition of liquid coolant to it.