Na(3)Co(NO(2))(6) has been investigated as a new reagent for the nitrosation of various substrates containing an amino functionality. Reactions took place in an aqueous solution of the reagent. The pH of the reaction mixture remained in the range 4.3-5. Thus, hydrazides were transformed to the corresponding acyl azides, and the reactions with arenesulfonyl hydrazines afforded arenesulfonyl azides. Treatment of aromatic amines with Na(3)Co(NO(2))(6) gave 1,3-diaryltriazenes in excellent yields; coupling of the initially formed diazo compound to the electron rich aromatic ring was also observed. Nitrosation of aliphatic amines was not possible due to complex formation with the reagent.