The pyrolysis of TNT and other explosive compounds generates signature compounds that can be efficiently detected by electrochemical sensors. This concept was implemented in the development and testing of a sensor probe for the Site Characterization and Analysis Penetrometer System (SCAPS) for the in situ detection of TNT, RDX, HMX, and other nitrogen-containing soil contaminants. This paper describes the results of laboratory studies and field tests conducted to determine the feasibility of employing electrochemical sensors for detecting subsurface explosives contaminants. A method for the in situ pyrolysis of explosives contaminants in soils was developed, and laboratory tests determined that electrochemical sensing of the pyrolysis products was sensitive, selective, reversible, and capable of broad dynamic range. A penetrometer probe that accommodates the electrochemical sensors (including power supply and signal conditioning electronics), the pyrolyzer unit, the pneumatic components, and geophysical sensors for soil classification was designed and fabricated. Results of tests conducted at the Louisiana Army Ammunition Plant during September 1994, which demonstrated the performance of the SCAPS sensor under actual field conditions, are presented.