An optical sensor system has been developed for the autonomous monitoring of NO 2 evolution in energetic material aging studies. The system is minimally invasive, requiring only the presence of a small sensor film within the aging chamber. The sensor material is a perylene/PMMA film that is excited by a blue LED light source and the fluorescence detected with a CCD spectrometer. Detection of NO 2 gas is done remotely through the glass window of the aging chamber. Irreversible reaction of NO 2 with perylene, producing the non-fluorescent nitroperylene, provides the optical sensing scheme. The rate of fluorescence intensity loss over time can be modeled using a numerical solution to the coupled diffusion and a nonlinear 4 chemical reaction problem to evaluate NO 2 concentration levels. The light source, spectrometer, spectral acquision, and data processing were controlled through a Labview program run by a laptop PC. Due to the long times involved with materials aging studies the system was designed to turn on, warm up, acquire data, power itself off, then recycle at a specific time interval. This allowed the monitoring of aging HE material over the period of several weeks with minimal power consumption and stable LED light output. Despite inherent problems with gas leakage of the aging chamber we were able to test the sensor system in the field under an accelerated aging study of rocket propellant. We found that the propellant evolved NO 2 at a rate that yielded a concentration of between 10 and 100 ppm. The sensor system further revealed that the propellant, over an aging period of 25 days, evolves NO 2 with cyclic behavior between active and dormant periods.5