Mn(III) has great potential for the oxidation of organic pollutants. In permanganate/bisulfite system, Mn(III) would form and react with target organics within seconds. Fast reaction makes revealing the reaction mechanisms difficult. In comparison, Fe2+/Mn2+/sulfite system can slowly release Mn(III) and facilitate the investigation of reaction mechanisms. Herein, N,N-diethyl-3-methyl benzoyl amide (DEET) was used as the target pollutant to comparatively study the oxidation selectivity of Mn(III) and peroxymonosulfate (PMS)/Co(II). By analyzing the transformation products (TPs) of DEET and Mn(III) consumption, we found that oxidative mechanism mainly existed in PMS/Co(II) process, whereas oxidative and non-oxidative mechanisms coexisted in Mn(III) oxidation process. The quantitative analysis of TPs illustrated that the peak concentration of dealkylation TP (TP-163) (the integral part of the relative molecular mass) and acetylation TP (TP-205) during Mn(III) oxidation process was 1.3 and 1.7 times higher than that in PMS/Co(II), respectively. Meanwhile, the peak concentrations of mono-hydroxylation TP (TP-207-I) in Mn(III) oxidation was lower than that in PMS/Co(II). Compared with PMS/Co(II), Mn(III) attacked preferentially the N-ethyl group during DEET degradation process. Toxicity assay suggested that Mn(III) oxidation technology was more effective on eliminating the toxicity of DEET than PMS/Co(II).