The movement of colloid-sized particles through the vadose zone has important water-quality implications, yet the mechanisms that govern colloid mobilization are poorly understood. Results of our laboratory column experiments demonstrate that moving air-water interfaces associated with a downward-propagating drying front are capable of scouring kaolinite colloids from the surfaces of quartz sand. The efficiency of air-water interfaces in capturing the immobile colloids increases with increasing drying-front velocity and decreasing porewater ionic strength. A model that approximates the sand pack as a bundle of capillary tubes and that links pressure-head changes during drainage to the rates of air-water interface movement through the tubes reproduces the time-series data on effluent-kaolinite concentrations.