Abstract: Electrical resistance heating (ERH) was used for the thermal remediation of two sites at Redstone Arsenal (RSA) near Huntsville, Alabama. Both sites feature karst limestone with heavy clay overburden. The two sites are located about five miles (10 km) apart, but differ considerably in the characteristics of the limestone. At site RSA-053, the heavy clay is underlain by very hard, uniform limestone with no apparent voids or soil inclusions. At site RSA-096, the limestone is riddled with soil-filled and water-filled voids; voids are also observed in the vadose zone soil. The largest water-filled void in the RSA-096 treatment volume appeared to be about 40 ft by 24 ft (10 m by 7 m) with a height that varied from 3 ft to 12 ft (1 m to 4 m). The limestone variation between the two sites provided a comparison of the effects of secondary porosity in a karst environment. In each case, remediation included treatment of soil and extended up to 40 ft (12 m) into saturated limestone. RSA-053 required the remediation of chlorobenzene; RSA-096 required the remediation of trichloroethene (TCE). The remediations targeted regions where dense non-aqueous phase liquid (DNAPL) was considered very likely, with contaminant concentrations greater than 10% of the water solubility limit. Pre-treatment groundwater concentrations were as high as 204,000 µg/l chlorobenzene and 990,000 µg/l trichloroethene. DNAPL was successfully removed from the target regions as indicated by the groundwater concentration reductions that were observed. Groundwater in the large RSA-096 void had relatively low initial TCE concentrations and the concentrations decreased further during the remediation.