Although much research has been done into flocculation systems involving anionic microparticles, few systematic investigations employing cationic polymer microparticles (CPM) have been reported. In the present work, emulsifier-free polymerization was conducted via copolymerization of styrene with three types of cationic monomer. This technique allowed cationic polymer microparticles with well-defined structures to be prepared. Dynamic flocculation experiments showed that the effectiveness of CPM alone as a flocculant for fine clay particles is affected by CPM charge density and by particle size, in the range 0.2 to 0.4 mu m. A strong synergetic effect between CPM and anionic polyacrylamide with high molecular weight and low charge density was observed. This significantly reduced the dosage of polymeric flocculant required for effective flocculation. The best flocculation occurred at a value of the anionic polymer:CPM ratio of 1:8 (wt), with anionic polymer dosage approximately 0.06 wt% on clay. Electrostatic interaction between CPM and anionic polymer appears to be an important driving force for flocculation.