The first in situ Fe K-edge X-ray absorption spectroscopic (XAS) study is reported on the electronic and structural properties of a nitrosyl non porphyrin adduct absorbed on an electrode surface. The specific species was formed, in this case; by exposure of the water-insoluble complex [meso-tetrakis(p-methoxyphenyl)porphyrinato] iron(II) ([Fe(TMPP)]) adsorbed onto high-area carbon (Black Pearls, ca. 1000-1500 m(2) g(-1)) to aqueous acid solutions containing nitrite. The adsorbed nitrosyl-Fe(TMPP) adduct, denoted as [Fe(NO)(TMPP)](ads), displays remarkable stability over a rather wide potential range. The X-ray absorption near-edge structure (XANES) and the X-ray absorption fine structure (XAFS) are characteristic of an [Fe-II(TMPP)(NO)(OH2)] species, in agreement with assignments proposed for solution-phase nitrosyl iron porphyrin adducts.(1) The chemistry and biochemistry of molecular and ionic forms : to be the subject af intense of nitrogen oxides continues fundamental and technological studies. Interest in this area spans a wide variety of disciplines, ranging from atmospheric sciences and toxicology to electrocatalysis and neurobiology. In particular, nitrogen oxides, derived from the burning of fossil fuels and coal, represent one of the most serious threats to the environment and human welfare.(2,3) Yet, as discovered fairly recently, nitric oxide (NO) is essential for many biological functions,(4,5) including vasodilation and neuronal communication. A better understanding of the electrochemical properties of nitrogen oxides may be expected to contribute greatly to the rational design of effective electrocatalysts for selective reductions, to yield added-value nitrogen compounds from inexpensive and abundant sources, and to the further optimization of highly specific, highly sensitive sensors for in vivo detection of nitric oxide.(6,7).