A manikin testing and modeling approach was used to predict the heat strain while wearing a Prototype Steam Suit Ensemble (PSSE) or the current U.S. Navy Submarine Steam Suit Ensemble (SSSE) and working in a steam-filled environment (88 deg C, 100% relative humidity) or a training environment (24 Degrees C, 65% relative humidity). Both ensembles were tested on a thermal manikin to measure thermal resistances. Metabolic rates during walking at 1.8 m/s (4 mph) and wearing the PSSE or the SSSE were estimated using an empirical equation. The six cylinder thermoregulatory model (SCTM) was used to simulate human thermal responses and determine the heat endurance times. Results showed that the PSSE performance improves, relative to the SSSE. The PSSE 1) increases thermal resistance by 70%, indicating more protection from external heat load; 2) reduces metabolic rates due to reduction in weight and possibly less hobbling; and 3) increases the predicted heat endurance times to about 33 min in a steam-filled environment. The analysis is limited to heat stress only and excludes all other possible injuries, such as burn injury.