A simulant with a suitable surface and the correct thermal inertia, &bgr;=k&rgr;c, meets the primary requirement of accepting heat at the same rate as the skin. For protected skin, a simulant with a blackened opaque surface, &bgr;½=0.035 cal·cm−2·sec−½·°C−1, and equivalent depthx/2√&agr;=0.12sec12(&agr;=k/&rgr;c)is suitable. Such simulants have been made of fused silica and of an epoxy resin filled with aluminum powder. The detector was a butt welded copper‐constantan thermocouple, flattened to a thickness of 0.0015 cm, or a thermocouple of evaporated gold and bismuth films, ∼6000 Å thick. Results with water flow simulating blood flow in the skin indicate that an inert system simulates the response of skin rather well; if some adjustment must be made for varying blood flow it can be done experimentally or by calculation.