(1): Selenenylsulfides should be rapidly attacked at Se by TrxR selenolate: (i) Rates of selenolate attack upon the Se of a selenenylsulfide are orders of magnitude faster than those of thiolates (kR1 vs kT1); though there are no data for cyclic 6-membered systems, linear selenenylsulfides are ca. 10 4 times as electrophilic to selenolate attack at Se as disulfides are at S. (ii) In general, selenolates are ca. 100 times as nucleophilic as thiolates. (iii) Selenols are also 2-3 pKa units more acidic than thiols, and the acidity of RSeH in TrxR is still greater . This enhances the general reactivity of the TrxR selenolate as compared to thiol species that are more likely to be protonated. Taken together, this suggests good initial rates of exchange at Se between TrxR and a cyclic selenenylsulfide. Importantly too, (iv) nucleophilic attack on the selenenylsulfide at Se is kinetically much more favoured than at S, fixing the initial attack site as the selenium.