Two novel substituted thiadiazoles I (R = nonyl or 1-naphthylmethyl) were synthesized by Pechman condensation, converted to resp. II by reaction with AcCH2COOEt in H2SO4/EtOH, purified by recryst. from EtOH (yield about 65%), identified by spectroscopy and studied for antioxidative activity by using the assay of 2,2-diphenyl-1-picrylhydrazyl radicals. The thiadiazoles II showed a lower ability to remove the reactive O species than the thiadiazoles I.