T his paper reports on the use of particle size analysIS nd settling tests for characterizing the structure of metal coagulation floes and further identifying optimum conditions for their settling and dewatering. A velocity-size variation law was established for model floes created from colloidal clay particles suspensions with different initial clay concentrations using two coagulants [aluminium sulphate, Al-2(SO4)(3) and ferric chloride, FeCl3]. It allowed deduction of the volume fractal characteristic D-3 of each clay floc family and their settling performances. Floes also appeared to be made from three basic units, the size of which was estimated from particle size analyses performed at different stages of the coagulation/flocculation. All the D-3 values obtained were high, about 2.5, nearly independent of the initial clay concentration. In contrast, the floc and the basic unit sizes decreased with the clay concentration increase. More concentrated suspensions led to lower settling rates. Ferric floes appeared to be made from larger basic units than aluminium ones and to be less dense and less compact with lower settling rates. They were therefore thought to contain more water.