Southwest Sciences Inc. (SWS), in collaboration with the Southwest Research Institute (SwRI), will develop a reliable, ultra compact, low power diode laser multigas sensor to measure carbon dioxide (CO2), ammonia (NH3), oxygen (O2) and water vapor (H2O) concentrations in the presence of saturated and condensable water concentrations appropriate for NASA's portable life support system (PLSS). A high sensitivity optical absorption technique known as wavelength modulation spectroscopy will be used in the sensor. The system will be light weight (<1 kg), low power (1 W), and fast (minimum 1 Hz measurement rate). The specifications of the proposed multigas sensor will provide reliable gas concentration measurements to ensure extended operation of the PLSS during extravehicular activities (EVA). The combined Phase I and Phase II project will provide NASA with a prototype sensor that will provide the same gas concentration data with equivalent or better accuracy as the current GS-300 and GS-322 sensors with the addition of an ammonia measurement not currently available in the PLSS.