This study investigated C-13 and H-2 isotope fractionation associated with oxidation of three phthalate esters (PAEs) by radical species, including sulfate radical (SO4 center dot-) induced by heat-activated persulfate (PS) and hydroxyl radical (HO center dot) induced by UV/H2O2. For persulfate oxidation at pH = 2 and pH = 7, similar carbon isotope fractionation (epsilon(C)) but distinct hydrogen isotope enrichment factors (epsilon(H)) were observed. The UV/H2O2 reaction of three PAEs showed smaller epsilon(H) values in comparison with persulfate oxidation. The correlation of H-2 and C-13 fractionation (Lambda) allows to distinguish the persulfate oxidation (25.7 +/- 2.6) and UV/H2O2 oxidation (2.4 +/- 0.2) of diethyl phthalate (DEP) highlighting the potential of compound-specific stable isotope analysis (CSIA) to characterize chemical oxidation mechanism of PAEs. Moreover, study of radical quenching and CSIA were combined to explore the dominant radical species during persulfate oxidation of DEP. SO4(center dot-) was found to be the predominant radical at pH = 2. Both SO4(center dot-) and HO center dot contributed to DEP degradation at pH = 7 and HO center dot was estimated to have a contribution of 21-63% according to dual C-H isotope fractionation values. Carbon and hydrogen apparent kinetic isotope effects (AKIEs) (C-13-AKI= 1.017, H-2-AKIE = 2.41) obtained from dominating sulfate radical reaction of DEP both supported the hypothesis of C-H bond cleavage. Thus, carbon and hydrogen isotope enrichment factors clearly distinguish the different reaction mechanisms and hence, are a promising approach to improve understanding of radical species reaction pathways for chemical oxidation of PAEs.