Technical Abstract: Epoxidized soybean oil (ESO) was converted to a polysoap via a two-step synthetic procedure of catalytic ring-opening polymerization (PESO), followed by hydrolysis with a base (HPESO). Various molecular weights of PESO and HPESO were prepared by varying the reaction temperature and/or catalyst concentration. In addition, the counter ion chemistry was varied by changing the base used for saponification. The PESO and HPESO products were carefully characterized and identified using a combination of FTIR, 1H NMR, solid state 13C NMR, and GPC. The effect of HPESO polysoaps on the surface tension of water and the interfacial tension of water-hexadecane was investigated as a function of HPESO concentration, mol wt, and counter ion chemistry. HPESO polysoaps were effective at lowering the surface tension of water and the interfacial tension of water-hexadecane and displayed minimum values in the range of 20-24 and 12-17 dyn/cm, respectively, at concentration of 200-250 micrometers. Water-hexadecane interfacial tension was also calculated from measured surface tension data using the Antonoff, harmonic mean (HM), and geometric mean (GM) methods. Measured values agreed well with those calculated using the HM and GM methods, but not the Antonoff method.