The effectiveness of ice and freon based personal cooling systems on the ability to work in the heat while wearing fully encapsulating suits was examined. The study group consisted of nine healthy volunteers, mean age 28.8 years. The subjects walked on a treadmill in an environmental chamber for 10 minute intervals interspaced with 5 minute rest periods for a total of 45 minutes at a work rate equal to 30% of their maximum aerobic capacity while wearing a self contained breathing apparatus (SCBA) and shorts, shirts, socks, and shoes (control condition); the SCBA, shorts, shirt, shoes, gloves, boots, and a fully encapsulating United States Coast Guard chemical response suit consisting of teflon coated nomex (CRS condition); the SCBA plus CRS fitted with an ice based personal cooling system (ICE condition); or the SCBA plus CRS fitted with a freon based personal cooling system (FREON condition). The chamber temperature and relative humidity were 33.9 degrees-C and 82%. The effects of the ensembles on heart rate, rectal temperature, and mean skin temperature measured from six sites were monitored for 55 minutes, including a 10 minute post exposure recovery period. Body weight loss due to sweating was determined. Mean skin temperature, heart rate increase, and body weight loss were significantly reduced under the ICE and FREON conditions. The mean time required for rectal temperature to return to the baseline value during the recovery period was significantly shortened under the ICE and FREON conditions. The decreases in the heat stress parameters were similar for the ICE and FREON condition. The authors conclude both the ICE and FREON systems can effectively minimize the physiological stress associated with wearing fully encapsulating suits. Neither system offers an advantage over the other.