Movement of wetting and drying fronts through the vadose zone can mobilize and transport colloid particles but the mechanisms are not fully understood. We used mesoscale (mm−dm) fluorescence imaging to measure mobilization of 1.9 μm diameter carboxylate-latex microspheres during drainage in quartz sand. Experiments were performed at ionic strengths of 2−50 mM and drainage rates of 1.0−0.2 mL min−1. Colloids were mobilized and transported steadily at a sharp decrease in pore saturation marking the drying front. The mobilization rate varied directly with the initial immobile particle concentration. The mobilization rate constant varied inversely with ionic strength and directly with drainage rate. Peak mobile particle concentration at the drying front varied nonmonotonically, and the mobilization efficiency decreased with distance traveled by the drying front, at high ionic strengths. These findings constitute evidence for particle redeposition from the drying front as drainage progresses, which we propose is a key factor in the observed variations with ionic strength and drainage rate in the total number of particles removed during drainage. The measured outcomes of particle mobilization during a drainage event are sensitive to the distributions of immobile particles prior to drainage and dependent on the length scales over which the drainage event is observed.