Firefighters in Queensland are exposed to hot, humid weather conditions that contribute to the overall workload encountered during emergency operations. Responding to certain hazardous material incidents requires firefighters to wear fully encapsulated chemical protective suits for a maximum period of 20 minutes. The nature of these suits, combined with workload and environmental conditions, poses a potential heat stress problem for firefighters. This study evaluates the heat-induced physiological responses of firefighters while wearing fully encapsulated chemical protective suits in a series of controlled thermal environments. Heart rate, body (aural) temperature, blood pressure, fluid loss, and a rating of perceived exertion were measured to evaluate the effect of increasing ambient air temperature during the performance of standard tasks. The results of the study indicated that the significant increase in heart rate, body temperature, and blood pressure was directly related to the increase in air temperature. The research indicates that the recommended suit wearing time of 20 minutes provided adequate physiological protection under the research conditions.