Reaction of [FeO(tmc)(OAc)](+) with the free radical nitrogen monoxide afforded a mixture of two Fell complexes, [Fe(tmc)(OAc)](+) and [Fe(tmc)(ONO)](+) (where tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and AcO- = acetate anion). The amount of nitrite produced in this reaction (ca. 1 equiv with respect to Fe) was determined by ESI mass spectrometry after addition of N-I5-enriched NaNO2. In contrast to oxygen atom transfer to PPh3, the NO reaction of [FeO(tmc)(OAc)](+) proceeds through an Fern intermediate that was identified by UV-vis-NIR spectroscopy and ESI mass spectrometry and whose decay is dependent on the concentration of methanol. The observations are consistent with a mechanism - involving oxide(o1-) ion transfer from [Fe (tmc) (OAc)](+) to NO to form an Fe-III complex and NO2-, followed by reduction of the Fe-III complex. Competitive binding of AcO- and NO2- to Fe-II then leads to an equilibrium mixture of two Fe-II(tmc) complexes. Evidence for the incorporation of oxygen from the oxoiron(IV) complex into NO2- was obtained from an O-18-labeling experiment. The reported reaction serves as a synthetic example of the NO reactivity of biological oxoiron(IV) species, which has been proposed to have physiological functions such as inhibition of oxidative damage, enhancement of perorddase activity, and NO scavenging.