N-Substituted benzisoselenazol-3(2H)-ones are well known for their ability to mimic the activity of an important antioxidant enzyme glutathione peroxidase. They eliminate the excess of reactive oxygen and nitrogen species and restore the cells redox homeostasis. In the catalytic cycle they can be easily transformed to diselenides, which also exhibit these unique properties. In this work, we have synthetized a series of N-alkyl benzisoselenazolones and corresponding diselenides. The antioxidant activity of all obtained compounds was evaluated and compared. The connection between the antioxidant properties and the ability to inhibit cancer cells proliferation was evaluated by a cytotoxic activity assay using PC-3 and DU145 cell lines.