Ferrocenyl and ruthenocenyl derivatives of ebselen are found to be competitive inhibitors of mammalian TrxR, with IC50 of 4.2 ± 0.3 and 2.5 ± 0.3 µM, respectively. The inhibition of TrxR is selective over GR, and in the presence of excess GSH. They exhibit lower Km values than ebselen and the natural substrate, Trx. © 2021 Elsevier B.V.