Cyanobacteria may cause an increase in the amount of assimilable organic carbon (AOC) that can lead to extensive microbial growth in the water. The present study systematically examined AOC formation from the oxidation of a pure culture of Microcystis aeruginosa by ozone and the advanced oxidation process of ozone/hydrogen peroxide (O-3/H2O2) and also explored the major mechanisms by monitoring the change of the algal integrity and structural change of algal organic matter. The results show that AOC formation followed a rapid-slow pattern during the ozonation of the algal suspensions. The rapid formation of AOC was attributed to the release of highly bioavailable intracellular organic matter, and the slow increase in AOC was due to the reaction of the algal organic matter with molecular ozone and/or center dot OH. Compared to ozone alone, a remarkable increase in AOC at the same ozone exposure was observed during O-3/H2O2, in which the increase in AOC was mainly attributed to the reaction of center dot OH with algal organic matter, resulting in its molecular weight change. A significant increase in the AOC concentration after oxidation of algae in actual waters was also seen, and a slightly higher amount of AOC was formed during O-3/H2O2 compared to ozone alone. (C) 2016 Elsevier B.V. All rights reserved.