In this STTR effort, Los Gatos Research (LGR) and the University of Wisconsin (UW) propose to develop a highly-accurate sensor for high-purity oxygen determination. The analyzer, which is based on LGR's patented Off-Axis ICOS technique, will be capable of rapidly quantifying high-purity oxygen (95 100 %) with very high accuracy (better than ? 0.03 %), minimal calibration, and no zero drift. Moreover, the sensor will require no consumables and be sufficiently compact and robust for deployment aboard the International Space Station (ISS). In Phase I, LGR and UW successfully demonstrated technical feasibility by fabricating a prototype that quantified high-purity oxygen with a precision of ? 0.017 % and a 24-hour drift of less than 0.05 %. The analyzer distinguished a 0.1 % change in highly pure oxygen and provided a linear response (R2 = 0.999997) over a wide dynamic range (0 100 % oxygen). The prototype was found to be accurate to 0.07 % by testing it at NASA Johnson Space Center on oxygen purified by the Cabin Air Separator for EVA Oxygen (CASEO) project. Due to the success of this program, LGR released a commercial O2/CO2 analyzer for environmental applications. In Phase II, LGR and UW will refine the measurement strategy, miniaturize the hardware, ruggedize the analyzer, and test the resulting instrument. The measurement strategy will be improved to reduce long-term drift and extended to include other species (H2O, O2 isotopes, N2). The hardware will be modified to meet the technical requirements for deployment aboard the ISS (e.g. power, size, weight, and environmental specifications). The prototype will be manufactured and tested to empirically determine its accuracy, precision, linearity, long-term drift, and time response. Finally, the Phase II instrument will be delivered to researchers in the Life Support and Habitability Systems Branch at NASA Johnson Space Centers for characterization of high-purity oxygen generators.