Two-phase microchannel heat exchangers for cooling integrated circuit (IC) dies and cooling systems employing the same are disclosed. The heat exchangers include thermal masses having a plurality of microchannels formed therein. In one set of configurations, the IC die is coupled to a thermal mass having a plurality of open microchannels such that a hermetic seal is formed between the die and the bases of the microchannel walls, thus forming a plurality of closed microchannels. In another set of configurations, a separate microchannel heat exchanger is thermally coupled to an IC die and operatively coupled to the IC die via coupling to a substrate on which the IC die is mounted. The microchannel heat exchangers may be employed in a closed loop cooling system includes a pump and a heat rejecter. The microchannels are configured to support two-phase heat transfer using a working fluid such as water.