An experimental apparatus for investigating the injection of nitrogen gas and water into the base of a steel tank containing molten tin has been developed. A first set of experiments based on only gas injection has been used to develop a diagnostic technique using continuous high-energy X rays and digital imaging to observe the mixing process and to measure local and average void fractions in the test section as a function of time and space. This unique application of real-time, high-energy, X-ray imaging has been used to generate two-dimensional mappings of the chordal-average void fraction with spatial resolution corresponding to a 0.43-mm2 cross-sectional area perpendicular to the X-ray path and time resolutions of <5 ms. Void fraction measurements with superficial gas injection velocities from 0.07 to 0.14 m/s into a 0.08-m-deep pool of 683 K molten tin indicate that the time and spatial average integral void fraction at these gas injection rates is relatively constant, in the range from 0.26 to 0.31. Similar injections into pools of 0.14- and 0.15-m depths have also exhibited relatively constant average integral void fractions in the range from 0.18 to 0.26. These values are in good agreement with past integral experimental measurements in mercury, Wood's metal, and molten steel.