3H-Spiro[1,3-benzothiazole-2,3'-indol]-2'(1',H)-ones 3a-c and 4a-e were synthesized from treating the 5-substituted 1H-indole-2,3-diones with 2-aminothiophenol in ethanol. The structures were confirmed by elemental analyses, spectrometry (IR, H-1 NMR, C-13 NMR, HSQC-2D and LCMS-APCI) and single crystal X-ray analysis. The new compounds were screened for their antioxidant activities such as the Fe3+/ ascorbate system induced inhibition of lipid peroxidation (LP) in liposomes, trolox equivalent antioxidant capacity (TEAC), scavenging effect on diphenylpicryl hydrazine (DPPH center dot), and reducing power. These compounds showed potent scavenging activities against DPPH center dot and 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS(center dot+)) radicals, reducing powers, and strong inhibitory capacity on lipid peroxidation. Compound 4a incorporating methyl both at R-1 and R-2 was found to be the most potent antioxidant described in this study. Compounds 3b and 4b were selected as representative compounds by the National Cancer Institute for screening against anticancer activity and these compounds were found to be cytotoxic against CNS cancer cell line SNB-75 in the primary Screen. (C) 2009 Elsevier Masson SAS. All rights reserved.