To address the NASA need for extravehicular mobility units (EMUs) with enhanced mobility, dexterity, lifetime, maintainability, and reliability for the International Space Station and for future missions to the Moon and Mars, Physical Optics Corporation (POC) proposes to develop a new Membrane Interferometric Carbon Dioxide Sensor (MICADS). This sensor integrates a fiber optic interferometer, sensitive to displacements of a small fraction of a wavelength, and a semipermeable membrane to selectively detect, identify, and quantify gases at concentrations at the parts per million (ppm) level. The MICADS will measure carbon dioxide level by means of a selectively permeable membrane, which induces small displacements in a diaphragm that can be measured by the interferometer. POC's light, compact, and reliable MICADS sensor will monitor and control the air processing to remove carbon dioxide from EMUs. It will not consume oxygen, and will produce only minimal heat. In Phase I, POC will develop and demonstrate the performance of the proof-of-concept MICADS. In Phase II, a prototype will be developed and submitted to NASA for testing.