The kinetics and mechanism of the reaction between nitric oxide and aquapentacyanoferrate(III) were studied in detail. Pentacyanonitrosylferrate (nitroprusside, NP) was produced quantitatively in a pseudo-first-order process. The complex-formation rate constant was found to be 0.252 +/- 0.004 M-1 s(-1) at 25.5 degreesC, pH 3.0 (HClO4), and / = 0.1 M (NaClO4), for which the activation parameters are DeltaH(+) = 52 +/- 1 kJ mol(-1), DeltaS(+) = -82 +/- 4 J K-1 mol(-1), and DeltaV(+) = -13.9 + 0.5 cm(3) mol(-1). These data disagree with earlier studies on complex-formation reactions of aquapentacyanoferrate(ill), for which a dissociative interchange (I-d) mechanism was suggested. The aquapentacyanoferrate(II) ion was detected as a reactive intermediate in the reaction of aquapentacyanoferrate(III) with NO, by using pyrazine and thiocyanate as scavengers for this intermediate. In addition, the reactions of other [Fe-III(CN)(5)L](n-) complexes (L = NCS-, py, NO2, and CN-) with NO were studied. These experiments also pointed to the formation of Fe(II) species as intermediates. It is proposed that aquapentacyanoferrate(III) is reduced by NO to the corresponding Fe(II) complex through a rate-determining outer-sphere electron-transfer reaction controlling the overall processes. The Fe(II) complex rapidly reacts with nitrite producing [Fe-II(CN)(5)NO2](4-), followed by the fast and irreversible conversion to NP.