The kinetics and mechanism of the thermal decomposition of the one-electron reduction product of [Fe(CN)(5)NO](2-) (nitroprusside ion, NP) have been studied by using UV-vis, IR, and EPR spectroscopy and mass-spectrometric and electrochemical techniques in the pH range of 4-10. The reduction product contains an equilibrium mixture of [Fe(CN)(4)NO](2) and [Fe(CN)(5)NO](3-) ions. The first predominates at pH < 8 and is formed by the rapid release of trans-cyanide from [Fe(CN)(5)NO](3-), which, in turn, is the main component at pH > 9-10. Both nitrosyl complexes decay by first-order processes with rate constants around 10(-5) s(-1) (pH 6-10) related to the dissociation of NO. The decomposition is enhanced at pH 4 by 2 orders of magnitude with protons (and also metal ions) favoring the release of cyanides from the [Fe(CN)(4)NO](2-) ions and the ensuing rapid delivery of NO. At pH 7, an EPR-silent intermediate I-1 is detected (nu(NO), 1695 and 1740 cm(-1)) and assigned to the trans-[Fe-II(CN)(4)(NO)(2)](2)- ion, an {Fe-(NO)(2)}(8) species. At pH 6-8, I-1 induces a disproportionation process with formation of N2O and the regeneration of nitroprusside in a 1:2 molar ratio. At lower pHs, I-1 leads, competitively, to a second paramagnetic (S = 1/2) dinitrosyl intermediate I-2, [Fe(CN)(2)(NO)(2)](1-), a new member of a series of four-coordinate {Fe(L)(2)(NO)(2)} complexes (L = thiolates, imidazole, etc.), described as {Fe(NO)(2)}(9). Other decomposition products are hexacyanoferrate(II) or free cyanide, depending on the pH, and precipitates of the Prussian-Blue type. This study throws light on the conditions favoring rapid release of NO, to promote vasodilatory effects upon NP injection, and describes new processes related to dinitrosyl formation and NO disproportionation, which are also relevant to the diverse biological processes associated with NO and N2O processing.