An efficient, rapid, simple, and inexpensive liquid phase microextraction method, called floating organic drop, combined microextraction with electrothermal atomic absorption spectroscopy was proposed for the determination of trace amounts of cobalt in several wastewater samples. Samples were prepared from Tehran oil refinery. In this technique, preconcentration of Co2+ was carried out by 2-nitroso-1-naphthol as complexing agent, from a 25-mL sample solution into a few microliters of n-octanol that formed by agitation with a stirring bar for a desired time. After coalescence of fine droplets on the surface, 10 mu L of the organic phase was withdrawn with a microsyringe, injected into a graphite tube, and cobalt content determined by electrothermal atomic absorption spectrometry. Under optimum conditions, the detection limit of Co2+ was 4 ng L-1, with a preconcentration factor of 155. The relative standard deviations for 0.1 mu g L-1 of Co2+ in water was 4.3% (n = 8). A dynamic linear range of 0.008-0.2 mu g L-1 with correlation coefficient R-2 = 0.994 was observed. The proposed method was applied successfully to the determination of Co2+ in oil refining wastewater samples.