Pyrolysis of hydrocarbon-contaminated soils offers the potential for rapid remediation without destroying soil fertility. Here we elucidate the fundamental mechanisms of pyrolytic treatment and advance understanding of the surface properties of pyrolyzed soils. Using thermogravimetry and evolved gas analysis, we identified the two stages of pyrolytic remediation. Desorption of light hydrocarbons is the dominant process for temperatures between 150 and 350 degrees C. Pyrolysis reactions dominate in the 400-500 degrees C range releasing gaseous products (hydrogen, methane, higher alkanes, and olefins) and forming a solid char. XPS analysis and partial combustion revealed that the char forms a layer coating the particles of treated soils. Since pyrolysis can effectively reduce the TPH of contaminated soils at temperatures below 500 degrees C, it avoids carbonate decomposition reactions that may lead to large soil pH increases and severe loss of fertility. This is a significant potential advantage over competing thermal processes that expose contaminated soil to temperatures above 500 degrees C.