The efficiency of coagulation and flocculation can be significantly improved through flocs breakage and re-growth under appropriate conditions. Investigation was carried out to examine the formation, breakage and re-growth of flocs formed by kaolin or the mixture of kaolin and humic acid with aluminum sulfate ('alum') or polyaluminum chloride ('PACl'). The reversibility and mechanism in the process of flocs breakage and re-growth were explored through conventional jar test procedure and continuous optical monitoring (PDA). There was full re-growth of flocs in kaolin water coagulated in charge neutrality (alum or PACl), which indicated a significant reversibility of flocs break-up process; the process displayed a distinct irreversibility in sweep flocculation. The addition of humic acid weakened the reversibility, and alum showed better shear resistance than PACl. The residual turbidity after breakage and re-growth was lower than before breakage when charge neutrality dominates the coagulate mechanism, but the result was reverse in sweep flocculation. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.