An experiment was performed to observe flow regimes and measure pressure drops of two-phase (liquid/vapor) flow and condensation in reduced gravity. Testing was conducted aboard the NASA-JSC KC-135 reduced gravity aircraft using a prototype two-phase thermal management system for large spacecraft. A clear section of two-phase line enabled visual and photographic observation of the flow regimes. The two-phase mixture was generated by pumping nearly saturated liquid refrigerant 114 through an evaporator and adding heat through electric heaters. The resultant two-phase flow was varied by changing the evaporator heat load, creating qualities from 0.05 to 0.80. Visual and photographic observation of vapor condensation was also made through a clear cover on the system condenser.During the flight tests, the experiment hardware was exposed to gravitational acceleration ranging from near-zero to 1.8 g's. Ground test simulations of the flight tests were performed before and after the KC-135 flights to generate a comparable one-gravity data base. The flight test results show that two-phase flow pressure drops can be predicted with reasonable accuracy for systems that will operate in reduced gravity by using either the existing Heat Transfer Research Institute (HTRI) method or the Friedel correlation.Following the testing of primary interest, described above, three additional tests, characterizing the thermal management system's performance, were successfully completed in reduced gravity. Throughout the entire test program the thermal management system performed as anticipated.