Flocs generated in various aluminum coagulants treatment exhibit difference in size, strength and structure. The formation, breakage and re-formation of flocs were investigated by using a laser diffraction instrument (Malvern Mastersizer 2000, Malvern, UK). Different Al coagulants - conventional Al salt (AICl3), polyaluminum chloride (PAC-1) and the purified polyaluminum chloride (PAC-2) - were investigated for the coagulation of humic acid (HA). The results showed the properties of HA flocs depend on which species of aluminum used. The monomeric and dimeric aluminum species (denoted as Al-mono) formed the largest and strongest HA flocs with longest growth time. The HA flocs formed by Al13O4(OH)(24)(7+) (Al-13 for short) is the smallest and weakest. The properties of HA flocs made it possible to investigate the relationship between the flocs growth process and aluminum species. (c) 2009 Elsevier Inc. All rights reserved.