The first in situ Fe K-edge X-ray absorption fine structure study is reported on the electronic and structural properties of a nitrosyl [meso-tetrakis(p-methoxyphenyl) porphyrinato]iron(II) adduct, [Fe(NO)(TMPP)], adsorbed onto a high-area carbon electrode. This adsorbed complex displayed a remarkable stability over a wide potential range, and multiple-scattering XAFS analysis yielded bond lengths of Fe−Np = 1.99, Fe−NNO = 1.74, and Fe−OH2 = 2.02 Å, and an Fe−N−O bond angle of 155°, characteristic of FeIINO porphyrin adducts.