Although chlorine radicals are strong atmospheric oxidants, their direct spectroscopic detection in the environment (gas phase) using conventional analytical techniques has not been reported. Herein, chlorine radicals (Cl-center dot), generated by the YAG laser photolysis (lambda = 355 nm) of Cl-2 in the gas phase, were captured by using 2,2-diphenyl-1-picrylhydrazyl (DPPH center dot). The product was identified as DPPH, wherein NO2 was substituted by Cl-center dot at the ortho-carbon on the picryl aromatic ring, and was characterized using ion attachment ionization-quadrupolemass spectrometry. This reaction mechanism is a rare example of the detection and characterization of radical species, where radical spin densities on trapping reagents and frontier orbital interactions play important roles.