The development and feasibility testing of a hybrid spacecraft heat rejection system that incorporates a single radiator capable of functioning as either a conventional space radiator or as a condenser in a refrigeration cycle is described. Emphasis is placed on development of the radiator/condenser (RC), which is considered to be the most critical component of the hybrid system. The selection, design and fabrication of candidate RC configurations are described together with preliminary parametric analyses necessary to establish pressure drop, heat transfer and flow stability characteristics. Verification testing in one-g and zero-g environments is described; the latter condition being obtained by means of a C-135 aircraft. The testing included flow visualization (i.e., high-speed photography) of the condensation processes in a parallel channel quartz tube system modeling the RC. Representative qualitative photographs are presented. Results indicate stable flow conditions prevail for both one-g and zero-g operation.