The goal of this research was to evaluate the parameters influencing the degradation of cylindrospermopsin by chlorine. Sodium hypochlorite was the source of chlorine used for the evaluation, with cylindrospermopsin obtained from a cell free extract of culture material. All experimental samples were of 100 ml volume in Milli-Q water, and undertaken in sealed Schott bottles. Prior to chlorination and after a given reaction period, 1 ml sub-samples were collected and immediately filtered through 0.45 um filters into a vial containing 25 ul acetic acid. Acetic acid was used to quench chlorine from samples for HPLC-MS/MS analysis of cylindrospermopsin as per method of Eaglesham et al. (1999). At the same time a 25 ml sample was collected for free chlorine determination using DPD pillows (Hach, 1993). Includes 8 references, tables, figures.