A superfamily of mononuclear iron proteins, originally named desulfoferrodoxin and neelaredoxin, has been identified by in vivo and in vitro studies as scavengers of the superoxide anion radical. These proteins, whose genes are present in all the so-far known genomes from anaerobes and in the microaerophilic pathogen Treponema pallidum, show not only a considerable amino acid sequence identity but, most importantly, a common active iron site, Fe[His(4)CysGlu], in the oxidized state which loses the glutamate ligand in the reduced form. The experimental evidence for the activity of these proteins as superoxide dismutases or as donor:superoxide oxidoreductases is discussed in this Commentary, giving particular emphasis to the neelaredoxin from the hyperthermophilic archaeon Archaeoglobus fulgidus.