Electron paramagnetic resonance (EPR) spectroscopy using sterically hindered amine is extensively applied to detect singlet oxygen ( 1 O 2 ) possibly generated in advanced oxidation processes. However, EPR-detectable 1 O 2 signals were observed in not only the 1 O 2 -dominated hydrogen peroxide (H 2 O 2 )/hypochlorite (NaClO) reaction but surprisingly also the 1 O 2 -absent Fe(II)/H 2 O 2 , UV/H 2 O 2 , and ferrate [Fe(VI)] process with even stronger intensities. By taking advantage of the characteristic reaction between 1 O 2 and 9,10-diphenyl-anthracene and near-infrared phosphorescent emission of 1 O 2 , 1 O 2 was excluded in the Fe(II)/H 2 O 2 , UV/H 2 O 2 , and Fe(VI) process. The false detection of 1 O 2 was ascribed to the direct oxidation of hindered amine to piperidyl radical by reactive species [e.g., • OH and Fe(VI)/Fe(V)/Fe(IV)] via hydrogen transfer, followed by molecular oxygen addition (forming a piperidylperoxyl radical) and back reaction with piperidyl radical to generate a nitroxide radical, as evidenced by the successful identification of a piperidyl radical intermediate at 100 K and theoretical calculations. Moreover, compared to the highly oxidative species (e.g., • OH and high-valence Fe), the much lower reactivity of 1 O 2 and the profound nonradiative relaxation of 1 O 2 in H 2 O resulted it too selective and inefficient in organic contaminant destruction. This study demonstrated that EPR-based 1 O 2 detection could be remarkably misled by common oxidative species and thereby jeopardize the understandings on 1 O 2 .