Results of tests in which metallic rods are burned in oxygen-enriched atmospheres often include the regression rate of the melting interface for the burning test specimen. This regression rate is used as an indication of a metallic materials' relative flammability when different metallic materials burn at the same test pressure and as an indication of a metallic materials' general ability to sustain burning under the test conditions. Most past methods for determining this regression rate have been based on visual interrogation which is costly, time consuming and often inaccurate. Inaccuracies typically result due to the specific method used (scale factors, optics, visualisation techniques, etc.), and often a limited access time with which to view the combustion event, or obscuration of the burning by condensed-phase products. An ultrasonic transducer has been fabricated to directly measure the regression rate of the melting interface established during the burning of metal rods, thus eliminating many of these problems. The transducer is described, typical results for an iron rod burning in pure oxygen are given along with a comparison of these results with regression rates obtained from visual interrogation. Excellent agreement is obtained between the regression rate obtained from the new ultrasonic transducer and regression rates obtained visually, thus demonstrating the transducer's usefulness.