The stability of colloidal silica in the presence of polyacrylamide has been studied by electroacoustic potential measurements and sedimentation testwork. Prior conditioning with AlCl3.6H(2)O or dodecylamine resulted in more effective action of the polymer. Studies carried out by transmission electron microscopy (TEM) showed that the aggregates formed by AlCl3.6H(2)O addition were branched and larger than those formed by DDA, under similar experimental conditions. The polyacrylamide(PAM) adsorption on the two types of aggregates was induced by different mechanisms which affected the characteristics and resistance of the flocs.