The effective specific air-water interfacial area (āi) in a sand-packed column was measured at several water saturations (Sw) using a surface-reactive tracer (sodium dodecylbenzene sulfonate (SDBS)) and a nonreactive tracer (bromide). Miscible displacement experiments were conducted under steady water flow conditions to quantify the retardation of SDBS resulting from its adsorption onto the air-water interface in a sand-packed column. A consistent trend of increased retardation of SDBS compared with the nonreactive tracer, bromide, was observed with decreasing Sw. The data for air-water surface tension measured at various SDBS concentrations were interpreted using the Gibbs model to estimate the required adsorption parameters. The retardation factors (Rt) for SDBS breakthrough curves were then used in combination with the estimated SDBS adsorption coefficient to calculate the āi values at different Sw. For the range of experimental conditions employed in this study, the retardation factor for SDBS ranged from Rt=1.07 at Sw=1.00 (Rt>1 due to SDBS sorption on sand) to Rt=3.44 at Sw=0.29 (which corresponds to āi=46 cm2/cm3). These values are in agreement with theoretical predictions and recently published data. Improvements needed to overcome the experimental limitations of the presented method are also discussed. ¿ 1997 American Geophysical Union