Abstract: In the cited flow injection method, analytes (acetic acid, caffeine, p-toluidine, 2,5-dinitrophenol or alprenolol) were dissolved in water and portions (25 µL) were injected into a stream of water which was then segmented with a stream of n-octanol. After passage through an extraction coil, a portion of the aqueous phase was separated with use of a cellulose nitrate/cellulose acetate (0.45 µm) or PVDF (0.22 µm or 0.1 µm) hydrophilic membrane and analyzed spectrophotometrically. The flow injection manifold was similar to that described previously (cf., Talanta, 1994 41, 1377). Partition coefficients were calculated by varying the phase flow rates. A simplified method involving only two measurements is described. The results agreed well with literature values obtained by the shake-flask method and the RSD were 15%. Phase separation efficiencies at various flow rates are tabulated. The assay time was 15 min.