With regard to heat stress, the limiting factor inherent in clothing ensembles is the total evaporative resistance. For the same work demands, the greater the evaporative resistance of the clothing, the lesser the ability to cool by sweat evaporation and hence the lower the environmental contributors to heat stress, especially water vapor pressure, must be. Knowing the evaporative resistance provides a means to compare candidate ensembles. Further, a value for evaporative resistance means that a rational method, such as the ISO Required Sweat Rate, can be used to assess the heat stress exposure. PURPOSE: Evaporative resistance of five clothing ensembles (cotton work clothes, cotton coveralls, and three coveralls of particle barrier (PB), micro-porous (MP) and vapor barrier (VB) properties) was determined empirically from wear tests. METHODS: For Yr1, the metabolic rate was held constant at 160 W/m2, and three levels of humidity (20, 50, 70% rh) were explored. Fourteen heatacclimated participants (9 men and 5 women) completed trials for all combinations of clothing ensemble and environment. In the Yr2 study, the humidity was held constant at 50% rh, and three levels of metabolic rate (114, 176, 250 W/m2) were explored. Fifteen heat-acclimated participants (11 men and 4 women) completed trials for all combinations of clothing ensemble and environment. The data from both years was combined and analyzed using ANOVA. RESULTS: Significant differences were found among ensembles (p<0.0001). As expected, VB coveralls had the highest resistance at 0.035 kPa m2/W. Micro-porous was next at 0.029; followed by PB and coveralls at 0.027. Work clothes was 0.026 kPa m2/W. Pair-wise comparisons adjusted for multiple comparisons were used to locate the differences among ensembles. VB was different from all others and MP was different from work clothes. From the year two study, we found that evaporative resistance decreased with increased metabolic rate. CONCLUSIONS: Data from both studies support that there are differences in evaporative resistances among different clothing ensembles. The decreased evaporative resistances in year 2 can be explained by the pumping action associated with increased work.