A hypersonic free jet wind tunnel has been designed and assembled at the University of Virginia to experimentally study rarefied flow phenomena. The purpose of this study is to provide a comparison of experimental and numerical results with theoretical/empirical predictions for an underexpanded free jet, which provides the test media for the tunnel. The hypersonic wind tunnel and an optical measurement technique have been developed in order to provide the necessary experimental measurements for this validation. Because the jet flow field contains both rarefied and continuum regimes, a previously developed planar laser-induced iodine fluorescence (PLIIF) measurement technique has been extended to measure temperatures in a range from 5 to 300K and pressures from 10-5 to 0.5 atm. The PLIIF technique was used to measure the temperature, pressure, and velocity for the free jet test section of the wind tunnel. Numerical predictions for the free jet are provided by the GASP computational fluid dynamics (CFD) code, and theoretical/empirical predictions are given by the Ashkenas and Sherman free jet relationships. The pressure, temperature and velocity measurements and calculations compared well, with the exception of the computed location of the Mach disk, possibly due to a breakdown of the continuum CFD model The comparison provides a validation of both the experimental and computational tools to be used to study mixed continuum/rarefied flows of interest to future aerospace vehicles. © 2002 by the American Institute of Aeronautics and Astronautics, Inc.