We report results of laboratory experiments on the co-transport of 137Cs by inorganic colloids composed of kaolinite. We find that under conditions of low pore water ionic strength, the kaolinite colloids significantly accelerate 137Cs transport through columns packed with quartz sand. Kaolinite mobility and the affinity of kaolinite for binding 137Cs diminish with increasing ionic strength. As a result, kaolinite exerts a progressively smaller influence on 137Cs transport as the ionic strength increases from 0.002 to 0.1 m. The 137Cs breakthrough data are used to test a model that incorporates advection-dispersion equations for the movement of kaolinite colloids, dissolved 137Cs, and kaolinite-associated 137Cs and mass transfer equations for kaolinite deposition, 137Cs adsorption by kaolinite, and 137Cs adsorption by quartz sand. The partition coefficient for 137Cs retention by kaolinite colloids and the first-order rate coefficient for kaolinite deposition vary in a discernible fashion with changes in ionic strength. The adsorption rate coefficient and the sorption capacity term of the second-order rate law taken to describe 137Cs adsorption to the quartz sand are independent of ionic strength; however; the magnitude of the desorption coefficient varies logarithmically with ionic strength. This work indicates the need to account for enhanced movement of sorbing solutes by inorganic colloids and provides a basis for quantifying the response of colloid-associated solute transport to changes in pore water chemistry. ¿ 1999 American Geophysical Union