Comparative osmotic compression experiments were performed on colloidal silica dispersions in the presence of various chloride salts at the same 0.01 M concentration with different counterions and highlighted the influence of ionic specificity on the resistance to water removal. These results were complemented with frontal ultrafiltration measurements that demonstrate modulation of the permeate flux according to the salt used. (c) 2005 Elsevier Inc. All rights reserved.