The objective of this research was to investigate the retention and transport behavior of GenX in five natural porous media with similar median grain diameters but different geochemical properties. Surface tensions were measured to characterize surface activity. Miscible-displacement experiments were conducted under saturated conditions to characterize the magnitude of solid-phase adsorption, while unsaturated-flow experiments were conducted to examine the impact of air-water interfacial adsorption on retention and transport. The results from surface-tension measurements showed that the impact of solution composition is greater for the ammonium form of GenX than for the acid form, due to the presence of the NH4 counterion. The breakthrough curves for the experiments conducted under saturated conditions were asymmetrical, and a solute-transport model employing a two-domain representation of nonlinear, rate-limited sorption provided reasonable simulations of the measured data. The magnitudes of solid-phase adsorption were relatively small, with the highest adsorption associated with the medium containing the greatest amount of metal oxides. The breakthrough curves for the experiments conducted under unsaturated conditions exhibited greater retardation due to the impact of adsorption at the air-water interface. The contributions of air-water interfacial adsorption to GenX retention ranged from ∼24% to ∼100%. The overall magnitudes of retardation were relatively low, with retardation factors < ∼3, indicating that GenX has significant migration potential in soil and the vadose zone. To our knowledge, the results presented herein represent the first reported data for solid-water and air-water interfacial adsorption of GenX by soil. These data should prove useful for assessing the transport and fate behavior of GenX in soil and groundwater.