In the present paper, the effects of suspension pH and charge density of anionic polyacrylamide (PAA) on flocculation behavior of two different natural stone suspensions (NSS) marble and travertine were investigated by settling rate and turbidity as indicators. The flocculation of NSS occurs even when the natural stone (NS) powders and the flocculants carry the same sign of the charge. The polymer bridging mechanism is responsible for flocculation of marble and travertine suspensions. The settling rate of NSS exhibits an optimal balance between the flocculating power of the polymer resulting from its expanded position in suspension due to its anionicity degree and the repulsive forces between the negatively charged NS powders and anionic flocculants. At high pH of 11, the settling rate of both natural stones increased significantly with each polymer except the 40% anionic polymer. In contrast, at low pH of 6, the supernatant turbidity of both suspensions was at the minimum level with no significant difference between them. However, when considering the settling rates, each polymer except the nonionic one showed different flocculation performances. The settling rate and turbidity results are analyzed in the light of zeta potential data to identify the mechanism of polymer uptake onto NSS. (C) 2004 Elsevier B.V. All rights reserved.