MtPrxQ B is a thioredoxin-dependent peroxidase. MtPrxQ B reduces fatty acid hydroperoxides with high catalytic efficiency. MtPrxQ B rapidly reduces peroxynitrite and is inactive on cholesterol hydroperoxides. MtPrxQ B is monomeric in solution both under reduced and oxidized state. Structural changes that occur upon MtPrxQ B oxidation are discussed.