Ex-situ biotreatment was the selected remedy for the source areas at a Superfund site (Site) impacted with chlorinated and non-chlorinated chemicals. The Contaminants of Concern (COCs) included chlorinated benzenes (mono-, di-, and trichlorobenzenes), 2-chlorotoluene (CT), naphthalene and nitrobenzene, with small amounts of chlorinated aliphatics. The biotreatment consisted of the excavation of source areas and treatment in windrows of approximately 1,400 cubic yards (CY). Initial active primary treatment of windrows occurred inside a building, followed by a passive secondary treatment outdoors on an asphalt pad. The primary treatment decreased the windrow COC concentrations to permitted levels (from a range of about 500-5,000 mg/kg to less than 240 mg/kg), allowing their transfer to the outdoor pad for secondary treatment. The primary treatment time varied from about 10 to 81 days (average of 23 days) and the secondary treatment varied from 10 to 140 days (average of 62 days). Factors that governed such variation included the windrow composition (source area soil type (sand versus sludge), COCs concentration and composition, salt content, and physical parameters (aeration frequency, mixing time, and temperature). The biotreatment end point was based on Synthetic Precipitate Leaching Procedure (SPLP) criteria and this leaching standard was met when the total COCs concentration in the windrow was typically at 50-80 mg/kg. The COCs evaporative loss during the primary biotreatment was captured and treated in a biofilter followed by adsorption onto an activated carbon bed prior to discharge to atmosphere through a stack. The results demonstrated successful large scale remediation of approximately 300,000 CY of source material by the ex-situ biotreatment technology.