A noninterfering diagnostic technique that employs monomeric hydrogen fluoride, HF, as a passive tracer gas has been developed. Infrared absorption by HF is used to infer turbulent mixing rates in inert and combusting flows. Data are presented on the turbulent diffusion of HF seeded streams of air, heated air, H2, and CO2into an outer coaxial air stream. Combustion of C2H2and air in a free jet configuration has also been investigated with the H F tracer. In most of these flow situations, the HF exhibited apparent rotational nonequilibrium. Development of the experimental technique and methods for data acquisition and analysis are discussed.