The flocculation characteristics of cationic starches with degree of substitution (DS 0.32-0.63) have been evaluated in 1.0 wt % kaolin suspension by spectrophotometry and colloid titration. Cationic starch is found to be an effective flocculant for removal of anionic suspension particles. Changes in the electrokinetics of kaolin as a function of pH were investigated in the absence of flocculant. The results show that kaolin in water exhibits a negative surface charge at pH > 2.5. The negative hydrophilic surface sites of kaolin are responsible for the adsorption of cationic starch molecules. The experimental data of the adsorption of cationic starch (DS 0.51) follow a Langmuir isotherm with maximum adsorption capacities of 16.89 mg/g. For the adsorption of cationic starch, chemical reaction seems significant in the rate-con trolling step and the pseuclosecond-order chemical reaction kinetics provides the best correlation for the experimental data. (C) 2007 Wiley Periodicals, Inc.