Abstract: In a dual phase cooling apparatus for an internal combustion engine, a condenser is interposed between a coolant fluid inlet, which receives coolant fluid as a mixture of its liquid and vapor phases from the engine, and a fluid outlet through which liquid coolant is returned to the engine. The condenser comprises a tube bundle mounted between two water boxes, at least one of which defines a phase separator interposed between the condenser fluid inlet and the condenser itself. One of the water boxes also carries a liquid reservoir interposed between the condenser itself and its fluid outlet. The structure may also include a pump for the liquid coolant fluid.