The goal of this report is to analyze the relationship between characteristic floc size and hydrodynamics in a mixing tank. The first question addressed concerns the relation between an average floc size and the viscous dissipation rate of kinetic energy. A first series of flocculation experiments were conducted in a mixing tank with two impellers (a Rushton turbine and a Lightnin A310 impeller) for equivalent dissipated power conditions. The average floc size is shown to depend on the global dissipation rate; it does not depend on the impeller type. However, the floc size distributions are significantly different for each impeller. The second question addressed concerns the dependency of the floc size on the history of mixing. A second series of experiments consisted of flocculation, breakup, and reflocculation stages. These experiments showed that the average floc sizes are similar after flocculation or reflocculation steps, but, once again, the floc size distributions can be very different with different impellers. The flocculation phenomena analyzed in this study mainly occur in the viscous subrange, with maximum floc size on the order of Kolmogorov microscale. (C) 2004 American Institute of Chemical Engineers.