The presence of microcystin toxins in drinking water is highly undesirable as they have the potential to adversely affect human health. Consequently, effective removal of these toxins from water is a major goal for water authorities. In this study, four microcystin analogues were chlorinated in two treated waters, and two of the analogues were chlorinated in deionised water. The oxidation of the microcystins was related to the chlorine exposure (CT) of the sample waters with the ease of oxidation following the trend: microcystin-YR > microcystin-RR > microcystin-LR >= microcystin-LA. This trend was in agreement with published data on model compounds and free amino acids. Values of CT of up to 25 mg min L-1 were required for oxidation of all microcystin analogues to below the World Health Organization guideline value of 1.0 mu g L-1. Results from this study indicate that for some water resources it is important to determine the speciation of the microcystin analogues to optimise chlorination practices. (c) 2006 Elsevier Ltd. All rights reserved.