Existing methods for the detection and measurement of volatile organic compounds (VOCs) in the vadose zone are often hampered by issues associated with ease of use, accuracy, and cost. As a result, there remains a need for inexpensive, minimally invasive, real-time instrumentation and sensor systems that can be used for characterizing or long-term monitoring of contaminated sites. A new sensor system to meet this need is under development at Boise State University, in collaboration with Washington State University. A miniature Ion Mobility Spectrometer (IMS) has been fabricated from a machinable ceramic material and has been shown to have a resolving power comparable to a commercial IMS. The IMS system will be packaged in a probe housing for deployment using direct push methods and will be ultimately equipped with water content, temperature, and pressure sensors. The proposed system is designed for use in multi-probe arrays and features wireless transmission of data directly to the user. To our knowledge, an in-situ IMS for detection of subsurface gaseous VOCs has not been previously developed. VOCs in Unsaturated Soils If a VOC enters unsaturated soil as a result of a spill, it can be present in four different phases: gas (air), water, solid (soil), and as a free phase liquid. Partitioning