Compressible two-phase (liquid/gas) jets have been proposed as a means of protecting the chamber walls in high-yield, low repetition rate, Z-Pinch IFE reactor systems. The aspect ratio (height-to-thickness/diameter ratio) of such jets is expected to be large, so that the void fraction may vary significantly along the flow direction. An experimental investigation was conducted to determine the effect of various design and operational parameters on the void fraction distribution within a planar, downward-flowing, two-phase (liquid/gas) free jet. An air/water jet with an initial cross section of 1.0 cm × 10.0 cm was used, and different liquid inlet velocities and gas-to-liquid volumetric flow rate ratios were tested. Local void fractions at different locations along the width and length of the jets were measured by gamma-ray densitometry. The results indicated that buoyancy caused significant slip between the two phases, leading to the conclusion that homogeneous two-phase flow models cannot accurately model the behavior of such jets. The data obtained in this investigation can be used to validate predictions of mechanistic models for jet dynamics and shock attenuation.