We synthesized a series of S-methyl-5-bromosalicylidene-N-alkyl substituted thiosemicarbazones and their cis-dioxomolybdenum(VI) complexes having long alkyl chains (propyl, butyl, pentyl, hexyl, and octyl) on thioamide nitrogen. The compounds were characterized by using analytical and spectroscopic methods. The structure of the complex with pentyl group (complex 3) as a representative molecule was determined by X-ray single-crystal diffraction method. The free ligand and their dioxomolybdenum(VI) complexes have been tested for in vitro antioxidant capacity by reduction of copper (II) neocuproine (Cu(II)-Nc) using the CUPRAC (CUPric Reducing Antioxidant Capacity) method. The ligands exhibited more potent in vitro antioxidant capacity than that of the complexes. The obtained trolox equivalent antioxidant capacity (TEAC) value of complex 1 (TEAC(CUPRAC) = 0.73) was higher than those of other complexes. Furthermore, the catalase activity and reactive oxygen species (ROS) scavenging ability of the free ligands and their complexes were determined, showing that complex 1 had significant scavenging activity for ROS. (C) 2017 Elsevier B.V. All rights reserved.