- This paper describes a new method for the passive sampling of groundwater colloids using a multilayer sampler (MLS). It is based on the use of dialysis cells with large pore size (10 μm) membranes that are in dynamic equilibrium with the mobile colloid and liquid phases in the aquifer. Under laboratory conditions, the dialysis cells reached equilibrium with a suspension of latex microspheres (5 mg/L) after 44−100 h and with a suspension of kaolinite (16−41 NTU; 20−50 mg/L) after 50−180 h. No fractionation was detected in the particle-size distribution between the kaolinite suspensions inside and outside the dialysis cells. Field profiles, obtained under natural gradient flow conditions in a sand and sandstone aquifer, showed large variability (up to 1 order of magnitude) in the colloid content within profiles (e.g., variation of 7 NTU (∼45 mg/L) between cells located at a vertical distance of 40 cm) and between them. The predominant colloidal particles found in the cells were aluminosilicates, CaCO3, silica, and organic matter. The membranes are suitable for sampling groundwater colloids over long periods of time, at least 36 days, in very turbid solutions (up to 50 NTU; ∼550 mg/L).