The aim of this paper is to show that the ESA (electrokinetic sonic amplitude) technique can be used to study the flocculation between cationic polyelectrolytes and negatively charged silica particles. The ESA signal was Measured when cationic polyacrylamide (CPAM) or cationic polyamine (POL) was mixed with silica particles. The dynamic mobility of the formed flocs was calculated from the corrected ESA signal. The amount of nonadsorbed polyelectrolyte was determined by viscosity. The flee sediment volume was also measured. It was found that the polyacrylamide molecule was more efficient than the polyamine molecule in neutralizing the charges on the silica particles due to the longer CPAM molecules and the larger distance between the CPAM charges which was in better accordance with the distance between the silica charges compared to POL.