In situ thermal desorption (ISTD) is a remediation technique that increases the effectiveness of soil vapor extraction through the simultaneous application of heat and vacuum. In this study, ISTD was applied to remove a chlorinated solvent source from unsaturated soil beneath an existing aboveground infrastructure. A material flow analysis (MFA) was applied for the first time to assess the effectiveness of ISTD in the vadose zone and to reveal the total emission of chlorinated solvents into the environment before and during remediation. The principle of matter conservation used in MFA enabled the quantification of chlorinated solvent flows in all matrices affected: soil, groundwater, and soil vapor. The MFA results revealed that the mass removed by ISTD was similar to the mean chlorinated solvent mass estimated to have been present in the soil before remediation, indicating high effectiveness in contaminant removal. The remediation target value in soil vapor was achieved after 9 mo of remediation, demonstrating the time efficiency of ISTD for this particular site. The MFA additionally provided an overview of the processes and contaminant transformations occurring in the soil, water, and air compartments during the course of remediation.