This article presents the preparation and in vitro biological activities of new 5'-arylchalcogeno-3-aminothymidine derivatives as antioxidants (inhibition of lipid peroxidation, scavenging of the free radical 2,2-diphenylpicrylhydrazyl and demonstration of a thiol peroxidase-like activity) as well as antitumoral agents against bladder carcinoma 5637. The chalcogeno-aminothymidines presented prominent activity in the tests for both biological properties, showing a direct relation with the chalcogenium atom.