Development of a suitable EVA suit for use on Mars will be a significant technological challenge. A particular concern is the excessive weight of existing planetary spacesuit designs. Mars has approximately one-third of the Earth's gravitational pull. Therefore, heavy suits will significantly hamper effective EVA operations. A suit design investigated by research groups from Stanford University and U.C. Berkeley uses semi-permeable membranes as a passive thermal control system. This design replaces the bulky active thermal control systems in more traditional spacesuit designs by working with the natural thermal control mechanisms of the human body. This idea is only possible due to the unique Martian atmosphere and the normal way in which the human body regulates its own temperature via sensory feedback to the brain, sweat, and regulation of blood flow.A mathematical model has been developed to simulate the thermal response of a human astronaut wearing the Mars suit during an EVA, and interactions with the Martian environment. Human metabolic output for various activities in reduced gravity is estimated from Wickman's experimental results. The suit has been shown to effectively control the body temperature on Mars using this heat-balance model. A similar model is used to explore the suit's ability to control body temperature in the Arctic on Earth. Results for a variety of simulated EVA excursions are presented.