Tetranitromethane (TNM) is examined for its suitability to modify amino acid residues in peptides, polymers, and proteins. It is shown that the reagent can be highly specific for the nitration of tyrosyl residues. Selective modification is achieved readily because reaction conditions are very mild. The reaction can be quantitated easily by spectrophotometry because nitrotyrosine absorbs in the visible region of the spectrum, and by amino acid analysis. Further, other products of the reaction, nitroformate and protons, can also serve as a means of quantitation. Under the usual reaction conditions TNM also oxidizes sufhydryl groups. TMN forms a charge-transfer complex with phenoxide ion, in aqueous solution, thus accounting for the pH-dependence of the reaction. An intermediate sulfenyl nitrate is formed which, in the presence of excess thiol, that is, low molar excesses of TNM, gives the disulfide and nitrite ion. TNM can be highly specific for tyrosyl modification when it is employed under the mild conditions described. Nitration with TNM is used successfully to study tyrosyl residues in many enzymes and proteins.