The deposition and release of colloidal contaminants in groundwaters may be important with respect to the environmental fate of contaminants and the remediation of subsurface contamination. The objective of this paper is to present results of laboratory studies that address the effect of ionic strength on model colloid (polystyrene spheres) deposition and release. Solutions of different ionic strength at near-neutral pH were pumped through a horizontally oriented, 40 cm long column of 0.46 mm silica sand at a velocity of 1 m per day. Pulse inputs of 1.09 μm fluorescent particles were made and the column effluent was continuously monitored with a fluorometer. After complete breakthrough of the pulse, the solution ionic strength was reduced and the subsequent effects on particle release were observed. Deposition results show evidence of the effects of hydrodynamic chromatography and retardation. As expected, deposition increases with increasing ionic strength. The extent of release due to ionic strength reduction depends on the magnitude of the change in ionic strength and the extent of prior deposition.