Provided is an ordered step-wise method for the chlorination and photodestruction of MC-LR contamination of an aqueous liquid such as a potable water supply infested with blue-green algae (cyanobacteria). It has been found possible to usefully chlorinate the water at lower chlorine:MC-LR ratios as compared to known methods and then to irradiate the water with ultraviolet radiation to photolyze the chlorinated MC-LR. The methods incorporate the prechlorination of a cyanotoxin, more advantageously a microcystin, and most advantageously MC-LR, to enhance UV-induced photodegradation of this compound.