Two types of helicopter passenger immersion suit constructed from either a vapour permeable or vapour impermeable material were assessed in terms of their wearers' comfort, their vapour transmission and the moisture accumulation in undergarments. Environmental air temperatures of 21°C and 30°C were compared to simulate dimerent helicopter operating temperatures, and the effects of wearing each type of suit. At 21°C the suits' performance were comparable in terms of increase in undergarment weight, sweat loss, percentage sweat evaporated, effect on rectal temperature and increase in mean skin temperature. At the higher temperature the permeable suit performed significantly better. There was less body weight loss, less sweat uptake into the undergarments and a greater amount of sweat being evaporated. Rectal temperatures and thermal comfort, however, were similar. At higher levels of thermal stress there may be an advantage in user acceptance and minimal decrement in reduction in cold water survival time from dampening if an immersion suit fabricated from a vapour permeable material is worn.