Peroxynitrite is a powerful cytotoxic species formed by the rapid reaction between nitric oxide and superoxide. The nitration of phenolics such as tyrosine by peroxynitrite is one of important paths peroxyntrite damaging biological systems. The effects of ferric citrate and ferric oxalate on the nitration of tyrosine by peroxynitrite have been studied. At physiological pH, ferric citrate and ferric oxalate have no effects on the reaction. At weakly acidic pH the reaction can be catalyzed by ferric citrate and ferric oxalate. The cause of the effects of pH on the catalytic activity of ferric chelates has been discussed.