A flotation–spectrophotometric method for the determination of Cu2+ has been developed. The flotation process is based on the formation of a ternary ion-associate between Cu2+, methylthymol blue (MTB) and cetyltrimethylammonium bromide (CTAB) which is floated at the interface of aqueous phase and n–hexane by shaking in a separating funnel. The aqueous solution is discarded and the adsorbed ion associate (Cu2+–MTB–CTA+) on to the wall of a separating funnel is dissolved in a small volume of methanol solvent and its absorbance is measured at 556 nm. The apparent molar absorptivity (ε) of the ion associate was determined to be 6.1×104 L mol-1 cm-1. The calibration curve was linear in the concentration range of 10-400 ng mL-1 of Cu2+ with a correlation coefficient of 0.9994. The limit of detection (LOD) was 6.7 ng mL-1. The relative standard deviation (RSD) for 50 and 300 ng mL-1 of Cu2+ was 3.9% and 1.3% (n=7), respectively. The method was successfully applied to the determination of Cu in water samples from water treatment process of Ramin power plant, Iran.