Real-time neutron radiography is being evaluated for studying the dynamic behavior of two-phase flow and for measuring void fraction in vertical and inclined water ducts. This technique provides a unique means of visualizing the behavior of fluid flow inside thick metal enclosures. An air-water flow system was constructed to simulate vapor conditions encountered in a fluid flow duct Air was injected into the bottom of the duct at flow rates up to 0.47 /s (1 ft3/min). The water flow rate was varied between 0 and 3.78 /min (0 to 1 gal/min). The experiments were performed at the Pennsylvania State University nuclear reactor facility using a real-time neutron radiography camera. With a thermal neutron flux on the order of 106 n/cm2.s-1 directed through the thin duct dimension, the dynamic behavior of the air bubbles was clearly visible through 5-cm (2-in.)-thick aluminum support plates placed on both sides of the duct wall. Image analysis techniques were employed to extract void fractions from the data, which were recorded on videotape. This consisted of time averaging 256 video frames and measuring the gray level distribution throughout the region. The distribution of the measured void fraction across the duct was determined for various air-water mixtures. Details of the results of experiments for a variety of air and water flow conditions are presented.