New bis(isatins-thio/carbohydrazones) based on Schiff bases were prepared from terephthalaldehyde biscarbohydrazone and 5-substituted isatins in the presence of a drop of sulfuric acid under reflux in ethanol. Terephthalaldehyde bis(thio/carbohydrazone) was synthesized by the reaction of (thio)/carbohydrazide and terephthalaldehyde in the presence of a few drops of acetic acid under reflux in ethanol. The structures of these synthesized compounds were determined using IR, H-1 NMR, and C-13 NMR spectroscopy and elemental analysis. The in vitro antioxidant activity of all the compounds was determined by the 1,1-diphenyl-2-picryl hydrazyl (DPPH center dot) free radical scavenging method. Compound 2 showed the best antioxidant activity.