NASA is developing new portable life support system (PLSS) technologies, which are being demonstrating in an unmanned ground based prototype unit called PLSS 2.0. One set of technologies within the PLSS provides suitable ventilation to an astronaut while on an EVA. A new component within the ventilation gas loop is a gas-to-water heat exchanger to transfer excess heat from the gas to the thermal control system's liquid coolant loop. A unique bench top prototype heat exchanger was built and tested for use in PLSS 2.0. The heat exchanger was designed as a counter-flow, compact plate fin type using stainless steel. Its design was based on previous compact heat exchangers manufactured by UTC Aerospace Systems (UTAS), but was half the size of any previous heat exchanger and one third the size of previous gas-to-liquid heat exchangers. The prototype heat exchanger is less than 40 cubic inches and weighs 2.57 lb. Performance of the heat exchanger meets the requirements and the model predictions at 200 lb/hr of water and 6 acfm of 4.1 psia nitrogen. The water side and gas side pressure drops are less than 0.8 psid and 0.5 inches of water, respectively. It transfers 20 watts of heat at nominal conditions with an effectiveness of 94%. I. Nomenclatureacfm = actual cubic feet per minute lb = pound force psia = absolute pressure in pounds per square inch psid = differential pressure in pounds per square inch Px = pressure transducer Tx = thermocouple Vx = valve