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U.S. military forces are currently using the Interceptor Body Armour (IBA) system which can increase human thermal stress when worn in and environments. This study investigated a spacer vest (SV) designed to distance the IBA from the wearer's skin surface, increasing evaporative cooling around the torso. A series of lightweight SV designed to be worn under the IBA was tested for thermal insulation (clo) and water vapour permeability (i(m)) on a sweating thermal manikin (TM). The TM was dressed in 3 configurations: with the U.S. Army Temperate Battle Dress Uniform (TBDU); with the IBA over the TBDU; and with the IBA over the various SV and the TBDU. TM results were used as input to a computer model predicting core temperature (T-c,degrees C), skin temperature (T-sk,degrees C), heart rate (HR,bpm), sweat rate, (SR,g/min), skin wettedness (SW,%), and total body water loss (WL,1). Output described responses when exposed to desert environments with air temperatures of 30, 40 and 50 degrees C during repeated, intermittent exercise (10 min rest/ 30 min walk). TM results showed thermal insulation increased and water vapour permeability decreased when IBA was worn over the TBDU. Use of a SV between the IBA and TBDU reduced thermal insulation and increased water vapour permeability. This translated into a theoretical increase in whole body evaporative cooling potential (i(m)/clo) of approximately 20% when wearing a SV compared to when wearing the IBA without a SV. Predictive model results showed thermo-physiological benefits when using a SV with lower SW at 30 degrees C, lower T, TA, HR, SR, SW, and WL at 40 degrees C and lower T-c at 50 degrees C.