A one-dimensional multiphase flow experiment was conducted to investigate the redistribution of nonaqueous-phase liquids (NAPL's) in the subsurface. NAPL, water and air contents were measured with a dual-energy γ-radiation attenuation apparatus, and NAPL and water pressures were measured with tensiometers that were inserted into the flow column. In the experiment, which used a sandy porous medium, both two-phase air-water and three-phase air-NAPL-water flow phenomena were measured. In the two-phase component of the experiment, water was drained from an initially water-saturated state by lowering the water table to a specified elevation. After the water drainage decreased to a negligible rate, the three-phase component of the experiment was initiated by infiltrating a 250-mL pulse of NAPL at the upper porous-medium boundary. Following a NAPL redistribution period, the water table was raised, lowered, and raised again. An objective of the experiment was to quantify the entrapment of air and NAPL under transient flow conditions. Analyses of the data showed that air was entrapped during NAPL imbibition, and NAPL and air were entrapped during water imbibition. The volume of entrapped fluids for similar saturation paths was reproducible, which suggested that fluid entrapment may be systematically modeled. It was concluded that to predict the movement of NAPL's in the subsurface, where the water-table elevation may fluctuate, constitutive relations among fluid saturations and pressures need to account for nonwetting fluid entrapment. The methodologies for conducting the measurements and associated error analysis are also given in this paper.