In order to provide a high- power density thermal management system for PEM fuel cell applications, a flow boiling microchannel evaporator plate has been developed that operates in a closed loop two- phase thermosyphon. The flow is passively driven by gravity, and the flow rate initially increases with increasing evaporation rate and then decreases after reaching a peak flow rate. A microchannel plate has been fabricated with 56 square channels that have a 1 mm x 1 mm cross- section and are 115 mm long. The working fluid, HFE- 7100, has been chosen due to its favorable saturation temperature at one atmosphere. Experiments have been conducted with the heat flux as the control variable. Measurements of mass flow rate, temperature field, and pressure drop have been made. The flow regimes are predominately bubbly and slug. The maximum heat flux observed, 32 kW/m(2), is an order of magnitude greater than that required in current fuel cells and is limited by a Ledinegg instability. Two- phase thermal hydraulic models give a reasonable prediction for the mass flow rates and wall temperatures using standard flow boiling correlations. This paper will thoroughly describe the performance of the two- phase thermal management system over a wide range of operating conditions.