Silver compounds and silver ions are used extensively in medical devices because of their wide-spectrum antimicrobial activity. In particular, nanoparticles of silver and silver (I) oxide show great promise for widespread usage in medical polymers and nanodrugs. Here, we demonstrate that a crystalline powder and a saturated aqueous solution of silver (III) oxide clathrate show much stronger antimicrobial activities and oxidative activities than silver (I) oxide.