Contamination of groundwater resources is a worldwide problem that threatens human health. Characterizing the type, location, and quantity of the contamination is the first step toward successful site remediation. Ground-penetrating radar (GPR) is one geophysical tool that provides information that may be used to derive the location and quantity of nonaqueous phase liquid contaminants (NAPLs). Because of the large contrast between the dielectric permittivity of water and NAPL, GPR is sensitive to areas in which NAPL displaces pore water. We have used numerical and physical models to investigate the GPR response to ultrathin layers of dense NAPL (DNAPL) trapped at a sand/clay interface. We have defined ultrathin as one-tenth of a wavelength or less at the dominant frequency of the radar signal. The numerical and physical models were in good agreement and both found an increase in reflection strength of 10% or more with partially DNAPL-saturated layer thicknesses as low as . The reflection strength generally increased with DNAPL-layer thickness reaching a 61% increase with a layer thickness of . These results were especially pertinent to field investigations because they were often limited to lower frequency radar () and NAPL accumulations may lie well below the conventional radar resolution limit.