Perfluorooctane sulfonate (PFOS) has been found in wastewater treatment plants (WWTPs) and in surface water as a result of domestic uses of textiles, electronics, and surfactants. The detection of PFOS in the aqueous environment has been linked to hazardous biological effects including estrogenicity and genotoxicity. To provide an alternative to conventional processes, one of the radical-based advanced oxidation and reduction processes being tested for treatment of refractory compounds in water, involves the use of an electron beam. Therefore, the aims of this study were to investigate the degradation efficiency of PFOS (100mg/L) by electron beam, to evaluate the predicted toxicity of the radiolysis products using the ECOSAR model, and to identify the radiolytic products of PFOS. As a result of using the ECOSAR model, the toxicity levels of by-products after electron beam treatment were reduced by decreasing the carbon-chain number of PFOS. The molecular structures of the radiolytic products were elucidated using authentic standards via liquid chromatography and tandem mass spectrometry, and by the interpretation of MS2 fragmentation patterns of each product using liquid chromatography with quadrupole time of-flight mass spectrometry (LC-QTOF-MS). In total, ten radiolytic products were confirmed by LC-MS/MS, HPLC, and IC data matching with commercial standards. The two radiolytic substances produced during irradiation with an electron beam were predicted by LC-QTOF-MS. This study led to an understanding of the role of electron beams in the transformation of parent compounds and to the decomposition products created when an electron beam is applied to treat perfluorinated compounds.