Mine ventilation systems are generally very complex, and many aspects of a mine ventilation network can be studied by conventional anemometer and smoke-cloud techniques. However, problems such as recirculation of return into intake air, leakage from adjacent mines, lost intake air, and unknown transit flow times through stoped areas call for new ways of analyzing airflow underground. Use of sulfur hexafluoride as a tracer gas can be an effective means of analyzing these ventilation problems. The Bureau of Mines is concerned with finding solutions to these problems in conjunction with its ventilation programs. Studies have been conducted by the Bureau in several mines in order to show the value of the sulfur hexafluoride tracer gas.