Recent laboratory and field studies indicate that colloids are capable of adsorbing and enhancing the transport of a variety of contaminants in groundwater. We examined the influence of kaolinite particles on 137Cs movement through sand columns. The times for transport of the main part of the 137Cs pulse decreased by over a factoor of 2 as the concentration of kaolinite in the column influent increased from 0 to 200 mg/L. Very early breakthrough of a relatively small amount of 137Cs was observed and attributed to the existence of sorption sites on the kaolinite that were kinetically controlled. This early breakthrough was essentially unretarded, whereas the initial breakthrough in the absence of kaolinite showed a retardation of about 15 pore volumes. Comparison of experimental observations with calculations from a transport model demonstrates that kaolinite deposition onto the sand was described best by an irreversible, first-order kinetics process and that retention of 137Cs by these clay particles was described by linear equations for two-site solute absorption. Uptake of 137Cs by the sand was represented by the Langmuir version of the two-site equations. This work indicates the need to account for facilitated transport of contaminants by inorganic colloids and provides a basis for quantifying the phenomenon. ¿ American Geophysical Union 1996.