AbstractThe antioxidant behavior of a series of new synthesized substituted indoline‐2‐ones and indolin‐2‐thiones was investigated in this study using an oxygen radical absorbance capacity assay (ORAC$_{ROO\mathop{\bullet}^{\!\!\!\!-}}$) and 2,2′‐azobis(2‐amidino‐propane) dihydrochloride (AAPH) as the radical generator; system generating superoxide anion radical, $O^{{\mathop{\bullet}^{\!\!\!\!-}}}_2$ (18‐crown‐6/KO2/DMSO), and the Fenton‐like reaction [Co(II) + H2O2 → Co(III) + HO• + HO−]. Measurements were done using fluorescence, chemiluminescence methods, and a deoxyribose assay based on the spectrophotometry method, respectively. The results obtained indicated that the examined indoline derivatives had effective activities as radical scavengers and may be considered as an effective source for combating oxidative damage. © 2005 Wiley Periodicals, Inc. Biopolymers 78: 171–178, 2005