The quantity and behavior of gases in metal and nonmetal mines were monitored to provide background information on feasibility of incipient fire detection systems. Monitoring was conducted during working and nonworking periods at intake air, return air, and working places. Gases generated during diesel operations and shot firing were of particular interest. Gases that were monitored were CO, co2, no, NO2, and so2. Submicrometer particulate concentrations were also monitored. Commercially available instruments were used and included nondispersive infrared, electrochemical, and chemiluminescent types. The ratios of the various gases generated during diesel operations and shot firing were similar to those expected from an incipient fire, except for the co2:nox ratio, which was sufficiently different to be considered for early fire warning. Samples taken near leaking bulkheads showed high co2 levels, and low CO levels, the reverse of anticipated behavior in coal mines.