Blooms of cyanobacteria can give rise to the production of toxins which contaminate drinking water sources. Among the oxidants and disinfectants typically applied in waterworks, chlorine has been found to be effective for the degradation of microcystins. In the present study, unknown second-order rate constants for the reactions of microcystin-LR (MC-LR), -RR and -YR with chlorine were determined over a wide pH range. It was found that an increase of pH has a negative effect on the microcystin degradation rate. Apparent second-order rate constant for the chlorination of MC-LR at 20°C varied from 475 M -1 s-1 at pH 4.8 to 9.8 M-1 s-1 at pH 8.8. From these apparent second-order rate constants, rate constants for the reactions of MC-LR with hypochlorous acid (HOCl) and hypochlorite (ClO -) were evaluated. Half-life times ranged from minutes at pH 6 to 1 h at pH 8 for a constant residual chlorine concentration of 1.0-0.5 mg l -1, typical of oxidation pre-treatment and final disinfection. Similar reactivity with chlorine was found for MC-RR and MC-YR. Therefore, chlorination is a feasible option for microcystin degradation during oxidation and disinfection processes, and can be applied in drinking water treatment in case of cyanobacterial toxin risk if the pH is kept below 8.