Importance of Fe (III) species in ferric chloride (FeCl3), polyferric chloride with basicity values (B, OH-/Fe3+ molar ratio) of 1.0 and 2.2 (denoted as PFC10 and PFC22) was investigated in terms of floc properties and membrane fouling during coagulation-ultrafitration process. The Fe (III) species distribution in these three coagulants was measured by a timed complexation spectroscopy method. Floc characteristics by FeCl3, PFC10 and PFC22 were studied using a laser diffraction particle sizing device. Moreover, SEM images and resistance analysis were conducted to explore mechanism of membrane fouling. The results indicated that the monomeric and polymeric species (Fe-a and Fe-b) increased as follows: FeCl3 > PFC10 > PFC22. FeCl3 had better coagulation performance in terms of turbidity, UV254 and DOC than PFC10 and PFC22. Meanwhile, FeCl3 produced largest flocs with loosest structure which were, however, weakest with best recoverability. PFC10 took the second place regarding to floc properties. The membrane fouling of PFC10 and PFC22 was more serious than FeCl3. SEM images and resistance analysis showed that cake layer resistance relative to the total resistance produced by FeCl3 was lowest due to the larger and looser flocs with high re-growth ability. Thus, more Fea and Feb could effectively reduce the membrane fouling. (C) 2014 Elsevier B.V. All rights reserved.