This U.S. Bureau of Mines report describes the results of research conducted in the experimental mine at Lake Lynn Laboratory on the response of fire sensors to mine fires, which include a slowly developing coal and/or conveyor belt fire, a rapidly burning liquid- fuel belt fire, and a liquid-fuel belt fire in the presence of diesel exhaust. Several mine fire sensors were evaluated with respect to sensor placement, spacing, and type. The data indicate that a smoke sensor will alarm several minutes before a carbon monoxide sensor and that, in the presence of diesel exhaust, only a prototype diesel-discriminating smoke sensor can successfully function without being sensitive to the diesel contaminants. The vertical placement of sensors in the entry near the fire was also shown to be critical in terms of alarm times. Additional data showed that variations exist in time to respond and level of response for two brands of electrochemical carbon monoxide sensors.