Although Al-13 polymer (AlO4Al12(OH)(24)(H2O)(12)(7+)) has been identified as an effective aluminum polycation in coagulation of natural organic matter (NOM), the coagulation behavior of Al-13 polymer largely depends on its preparation conditions and characteristics. The performances of pre-hydrolyzed Al-13 and in situ formed Al-13 in humic acid (HA) coagulation were comparatively investigated in this study. Floc properties, which were evaluated in terms of floc size, strength and compaction degree, were measured using a laser diffraction particle sizing device. Additionally, variations of these floc properties with coagulation time were also studied. The results showed that, AlCl3 showed the best HA removal efficiency of above 92% at pH 5.5-6.0, where in situ formed Al-13 was present. The preformed Al-13 species gave rise to better removal efficiency than AlCl3 across the pH range with the exception of pH 5.5-6.0. This indicated that both preformed and in situ formed Al-13 polymer could enhance the HA removal efficiency. Flocs formed by the preformed Al-13 were stronger, as reflected by the lower gamma` value of 0.53: while the gamma` value for the non-preformed Al-13 was 0.72. Additionally, the pre-hydrolyzed Al-13 species tended to produce more compact flocs with larger D-I values than the in situ formed. (c) 2011 Elsevier B.V. All rights reserved.