Significant amounts of dimers, oligomers, and polymers that are associated with adverse health effects are produced during frying. However, chemical bonds forming polymers have not been well understood. In this study, ester bonds, which were recently found to be one of chemical bonds producing polymers in oxidized biodiesel, were examined in soybean oil (SBO) oxidized by frying and heating at 175 ºC. Ester value increased as SBO oxidized up to day 3 and slightly decreased at day 4 indicating that esterification and hydrolysis concomitantly occurred under the frying and heating conditions used in this study. The 13C NMR spectrum showed new ester carbonyl carbon signals evidencing the formation of ester bonds. This study concluded that there were unidentified chemical bonds forming polymers other than ester bonds, and an attempt was made to find these chemical bonds using NMR spectroscopy. The reaction of oxidized SBO with acetyl chloride verified that proton signals at 3.61 and at 3.71 ppm were those of alcohols produced in oil oxidation. Although a previous study reported that ether bonds formed polymers in oil, no NMR signals indicating ether bonds were observed with SBO oxidized under the conditions in this study. Using the 1H NMR spectrum after the reaction with acetyl chloride, some uncertain assignments of proton signals of oxidized SBO were clarified as signals of alcohols, several previously assigned signals as alcohols were confirmed, and a new proton NMR signal was assigned to be an alcohol.