Dual-polymer flocculation with unmodified and ultrasonically conditioned flocculant is presented in this paper. In the experiments two types of suspensions, i.e. chalk dust and kaolin clay in RO water were investigated. In the former case the bridging mechanism was expected whereas in the latter case charge neutralization and charge patching mechanisms should be dominant. The measurements were carried out in a laboratory setup for on-line particle size distribution (PSD) analysis consisting of stirred vessel with turbine mixer, laser particle size analyzer Fritsch Analysette 22 and peristaltic pump. The flocculation kinetics is described in terms of PSD and mean particle diameter change in time. Authors proved that ultrasonic conditioning may be an effective alternative for a dual-polymer flocculation where two different flocculants are used. In some cases the mechanical strength of flocs, created by combination of unmodified and conditioned polymers, was improved which resulted in reduction of time needed to achieve the steady state of the system. (C) 2010 Elsevier By. All rights reserved.