Abstract: A complex extraction solvent composed of tri-n-octylamine (TOA) and n-butanol was used to separate N-hydroxysuccinimide (HOSU) from HOSU containing solutions (concentration:0.015~0.03 g·mL^-1). The optimal extraction system was obtained using a single-stage equilibrium apparatus. Effects of TOA volume ratio, oil to water volume ratio, extraction temperature and sailing-out agent on the distribution coefficients of HOSU between oil and water phases were investigated, and liquid-liquid equilibrium data was measured at 25℃. Infrared spectroscopic results show that TOA-HOSU complexes are formed via hydrogen and ionic bonding. A mathematical model based on mass balance is developed to describe extraction equilibrium and correlate experimental equilibrium data at 25℃. The results show that the calculated HOSU concentrations in the organic phase are in good agreement with experimental data.