Sludges are made of floes produced during the coagulation stage. Their dewatering not only depends on the equipment used but also on the chemical, structural and physical characteristics of flocs. In this study, the relation between floc structure and sludge dewaterability is investigated beside the influence of the operating conditions during coagulation. Eight synthetic clay flocs families are produced using clay suspensions with four initial solids concentrations ranging from 0.5 to 5 g L-1 and two coagulants (Al-2(SO4)(3) and FeCl3). Flocs structure is characterised by their fractal dimension and by the size of the basic units (clusters, aggregates) they are made from. Fractal dimension values about 2.5 are indicative of rather compact structures with slightly looser flocs obtained when using FeCl3. Except for clusters, larger in ferric flocs, the basic units sizes are similar whatever the coagulant used and they decrease when the solids concentration during coagulation is increased. Sludge dewaterability is estimated from the bound water content and the capillary suction time (CST) giving respectively information on the extent and on the rate of water removal. Ferric flocs contain about 20% less bound water but exhibit higher CST values and therefore higher resistance to water removal than alum ones. Flocs structure and sludge dewaterability are found linked. The sludge bound water content is found to decrease with the floc fractal dimension: less compact flocs contain more water but less bound water. And the capillary suction time is lower when sludges are made of smaller flocs. (C) 2004 Elsevier B.V. All rights reserved.