Abstract A cloud-point extraction (CPE) technique for the determination of a cyanobacterial hepatotoxin, nodularin-R, in aqueous media using a cationic surfactant, tricaprylmethylammonium chloride (Aliquat-336), was developed. Cloud-point phase separation of Aliquat-336 at ambient temperature was induced by the addition of sodium sulfate. The Aliquat-336/Na 2 SO 4 CPE system displayed large preconcentration factor, F C , for nodularin-R. At the operational CPE conditions, F C of 709.2 was achieved. Distribution constant, K D , of the distribution of nodularin-R between the surfactant-rich and aqueous phases of the CPE system was estimated to be (4.94±0.8)×10 3 . Coupled to liquid chromatography with UV detection, the CPE method offered a detection limit of 330 pg ml −1 (in freshwater)/1.3 ng ml −1 (in seawater) and a repeatability of 6.4% (in freshwater) ( n =7, P