The thermal, mechanical and morphological characteristics of three selected commercially-available, injection-moldable, high thermal conductivity (20 - 32 W/m.K), polyimide 66 (PA66) polymer composites from two vendors are characterized for possible heat exchange applications in electronic equipment. The fillers are found to consist of 10 μm diameter, 120 to 350 μm long fibers, made of carbon in two composites, and a hybrid combination of essentially carbon, oxygen, and silicon in the third composite. Fiber weight loading ranges from 63% to 69%. The hybrid, high-length fiber reinforced material overall displays superior mechanical properties (i.e., ultimate tensile, flexural and impact strengths, and flexural modulus) compared with the other two carbon-filled composites. For the hybrid-filled and one carbon-filled material (both having a thermal conductivity of 20 W/m.K), good agreement between mechanical property measurements and corresponding vendor data is obtained. For the material having the highest vendor-specified thermal conductivity (i.e., 32 W/m.K) and weight filler fraction (i.e., 69%), mechanical properties are up to 37% lower than corresponding vendor data. The heat transfer rates of parallel plate, cross-flow air-water heat exchanger prototypes made of the three PA66 materials are comparable to that of an aluminum prototype having the same geometry. Based on the combined heat transfer and mechanical property characterization results, the hybrid, long-fiber filled PA66 polymer composite appears to have the best combination of mechanical and heat transfer characteristics, for potential use in electronics heat exchange applications.