Volcanic research is a significant part of the "Earth Surface & Interior" focus area of the NASA Earth Science program. Of particular interest are sensors for measuring the chemical composition and emission rate of gases released by volcanoes, from magma to the atmosphere, both prior to and during eruptions. The vast majority of ground based plume observations concern sulfur dioxide (SO2), the third most abundant volcanic gas. Sulfur dioxide measurements provide valuable information concerning the masses and motions of underground magmas. Vista Photonics proposes an innovative self-calibrating optical technology for sensitive and selective in situ measurement of SO2 in volcanic gas plumes. The self-calibrating feature is intended to maintain the specified performance characteristics under harsh operating conditions. Phase I research will focus on demonstrating the feasibility of the innovative SO2 detector and outline the design of the Phase II prototype instrument. Successful completion of this program will lead to development of an inexpensive, compact, self-contained, lightweight SO2 measurement instrument suitable for use on a variety of ground based and airborne platforms including remotely operated aircraft and surface craft.