Nonstoichiometric polyelectrolyte complex dispersions (PECs) with different charge excess and hydrophobicity as well as different average hydrodynamic particle size were used to induce flocculation of oppositely charged silica dispersions. PECs were formed with poly(diallyldimethylammonium chloride) (PD) as polycation and poly(styrene-p-sodium sulfonate) (NaPSS) and poly(acrylamide-co-sodium acrylate) (PR2540) as polyanions. PD was used as single polymer flocculant too. Flocculation process was characterized by optical densities of supernatants OD500, by UV/VIS spectrometry, and determination of average floc sizes D(v; 0.5) as well as volume distribution of floc sizes by laser diffraction and determination of sedimentation velocity s by means of LUMiFuge (TM) 114. It was found that the reaction process between silica and the used flocculants could be divided into three intervals (destabilization, flocculation optimum, and restabilization) as it is known for all other polymer flocculants. For an effective flocculation of a charged substrate, both electrostatic as well as hydrophobic interactions play an important role. The interval up to the beginning of the flocculation optimum is mainly ascertained by electrostatic interactions (the charge density of the flocculant) but the broadness of flocculation optimum depends largely on hydrophobic interactions. Hydrophobic interactions also play an important role for shear stability of the formed flocs. (c) 2006 Wiley Periodicals, Inc.