The influence of precipitated calcium carbonate (PCC) characteristics on the flocculation process and flocs structure was investigated in the presence of a linear, high molecular weight, cationic polyacrylamide of high charge density. For that, a commercial PCC sample was used, either as supplied (original PCC) or after a washing process (treated PCC). The PCC samples were characterised, and the flocculation process, for different flocculant concentrations, was monitored using the light diffraction scattering technique (LDS). Flocs structure was evaluated by determining the mass fractal dimension of the flocs. It was found that the particle size of the original PCC was larger than that of the PCC submitted to the cleaning process. As a consequence, the optimum flocculant dosage was lower for the original PCC than for the treated PCC. Despite the kinetics of flocculation being independent of the differences in the PCC samples, the flocs compactness at the end of flocculation was higher in the case of the original PCC, resulting in more resistant flocs to shearing. The results showed that the PCC characteristics affect mainly the optimum flocculant dosage, the flocs structure and, thus, the flocs resistance and their reflocculation capability (c) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.