The location of chemical defects in single-walled carbon nanotubes (SWCNTs) can easily be identified as is presented in this communication. The method relies on the formation of Se nanoparticles via site-selective oxidation of H2Se, which works under soft conditions at room temperature and is able to reveal the defects even in individual tubes. The Figure depicts an AFM image of SWCNTs after H2Se exposure for 100 s.