Algal organic matter (AOM), especially intracellular organic matter (IOM), increases the levels of color and taste & odor, and promotes the formation of disinfection by-products (DBPs). This study isolated the IOM from Microcystis aeruginosa into the fractions with different molecular weight (MW), i.e., >100 kDa, 30-100 kDa, 10-30 kDa, and 3-10 kDa, and characterized them by techniques of dissolved organic carbon (DOC), UV absorbance and Excitation-emission matrix (EEM) fluorescence spectroscopy. After that, the removal efficiency of these IOM fractions by aluminum sulfate (Al-2(SO4)(3), alum) at different Al doses and pH were compared. The removal efficiency of IOM, as indicated by DOC, UV254, and OD680, increased consistently with higher Al doses, and the maximum removal was determined to be 99.7% for OD680, 51.4% for UV254, and 38.7% for DOC by alum at 5 mg/L. pH also showed effects, and the maximum removal was achieved at pH 6.5 with the maximum removal of 42.3% for DOC and 61.5% for UV254. The higher UV254 removal as compared to that of DOC indicated the superiority of alum to remove the aromatic IOM species. EEM spectra indicated the higher protein content within these IOM fractions, and the formation of complexes between Al and proteins involved in their removal. IOM with higher MW showed more significant removal of both DOC and UV254 than that with lower MW, and the sweep flocculation mechanism played an important role. Alum coagulation is effective and available for IOM removal in case of algae bloom and minimize its adverse effects on water safety thereafter. (C) 2017 Published by Elsevier B.V.