Abstract: Equations are presented for the calculation of partition coefficients determined by two-phase FIA. The equations were demonstrated with the injection of 20 µL metoprolol, p-hydroxybenzaldehyde, p-nitrophenol, phenol, propranolol and p-toluidine in CHCl3 or 0.15 M phosphate buffer of pH 5-6 into a carrier stream of CHCl3 or buffer and subsequently mixed in a PTFE reaction coil (2 m x 0.5 mm i.d.). The two phases were pumped at a total flow rate of 0.21-3.7 ml/min and the organic and aqueous phases separated under N2 using 0.2 and 0.22 µm PTFE and poly(vinylidene fluoride) membranes. The absorbance of the separated phases was determined by spectrophotometry. The product of carrier flow rate and peak area RSD was 0.87% (n = 11). Separation efficiencies of 80-90% and 40-50% were obtained for the hydrophobic and hydrophilic membranes, respectively.