The flocculation of an industrial furnish of TCF Kraft bleached pulp and ground calcium carbonate, induced by polyethylene oxide, and by the dual system formed by polyethylene oxide (PEO) and formaldehyde-phenol resin (PFR), was investigated and compared in order to study the flocculation kinetics, the flocculation mechanism and the properties of the formed flocs. A technique based on the measurement of the particle chord size distribution, called focused beam reflectance measurement, was used to carry out the research. The ability of the polyacrylate contained in the commercial ground calcium carbonate as a PEO cofactor was observed. Results confirmed the formation and the coacervation of the complex PFR-PEO, which loses its flocculant properties over time, confirming the complex bridging flocculation model. The shearing forces effect on the flocculation kinetics and on the properties of the formed flocs was also studied. (C) 2005 American Institute of Chemical Engineers.