Two-phase gas-liquid how experiments have been developed for use on NASA microgravity aircraft to allow for high speed measurement of void fraction, liquid film thickness and pressure drop as well as high-speed photography of the flow features. Numerous experiments were conducted in order to determine the effect of liquid and gas superficial velocities, tube diameter, liquid viscosity and surface tension on the occurrence of flow patterns in microgravity. The transition from bubble to slug dow was found to be affected by tube diameter for air-water and by changes in liquid viscosity and surface tension. The transition from slug to annular flow was not significantly affected by changes in tube diameter, liquid viscosity or surface tension. Void fraction based transition models were developed to predict microgravity flow patterns. Weber number based transition models were also evaluated.