Physiological stress in fire fighters caused by a turnout suit with and without a water barrier composed of a microporous membrane was measured during simulated fire fighting work. Eight experienced fire fighters (23 – 47 years old) were randomly exposed to three wear trials consisting of a standardized treadmill test of 25 min of hard work at Ta of 50 °C, and P of 1440 W/m2, interrupted by a 25-min break at Ta of 22 °C, and followed by 30 min of moderate work at Ta of 40 °C, and P of 258 - 177 W/m2. Body temperatures and heart rate increased steadily during the work periods up to the individual heat tolerance limits. The body temperatures were higher when the suit with a membrane was worn. The difference in rectal temperature was not significant, but the difference in mean skin temperature was statistically significant at the end of the hard work (p=0.05). More sweat condensated in the underclothing, and the subjective evaluation of physical exertion, thermal discomfort and sensation were higher for the suit with the membrane, but only slight statistically significant differences were found. The corrected thermal insulation (ITcorr) and the water vapor permeability (Me) measured with the aid of a sweating thermal manikin were higher for the suit without the membrane.