Colloidal Hg release and transport in soils is of significance due to its high Hg concentration and toxicity. Recent laboratory studies have suggested colloidal HgS is important vector for Hg dispersal from Hg-contaminated mining tailing. However the contributions by organic matter and Fe oxide colloids on Hg transport are not well studied. In this study, colloid mobilization and its effect on Hg transport were investigated in three contaminated soils: MN1 and MN2 (spiked with HgS and Hg(NO₃)₂ and aged for 5 years) and OR1 (contaminated soil from Oak ridge). Batch experiments were used to study the composition and Hg concentrations of water-dispersible colloids. Column experiments were conducted to examine the effect of ionic strength (0.05 and 5 mmol/L) and humic acid on colloid release from Hg contaminated soils during saturated conditions. Flow interruptions were performed to study Hg association with iron oxide colloids in the effluent. Concentrations of Hg in water-dispersible colloids in three soils were 1.6, 2.1, 4.4 times higher than in whole soils (< 2mm). Lowering the ionic strength in solution resulted in release of colloidal Hg from all three soils, with more from MN1 (0.1%, 17.7µg) than MN2 soils (0.3%, 61.1 µg). The results of this study should help better understand the role of colloids in Hg transport in contaminated soils.