We describe here nitric oxide dioxygenation (NOD) by the dioxygen manganese porphyrin adducts Mn(Por)-(eta-O-2) (Por(2-) = the meso-tetra-phenyl or meso-tetra-p-tolylpor-phyrinato dianions, TPP2- and TTP2-). The Mn(Por)(eta-O-2) was assembled by adding O-2 to sublimed layers of Mn-II(Por). When NO was introduced and the temperature was slowly raised from 80 to 120 K, new IR bands with correlated intensities grew concomitant with depletion of the v(O-2) band. Isotope labeling experiments with O-18(2), (NO)-N-15, and (NO)-O-18 combined with DFT calculations provide the basis for identifying the initial intermediates as the six-coordinate peroxynitrito complexes (ON)Mn(Por)(eta(1)-OONO). Further warming to room temperature led to formation of the nitrato complexes Mn(Por)(eta(2)-NO2), thereby demonstrating the ability of these metal centers to promote NOD. However, comparable quantities of the nitrito complexes Mn(Por)(eta(1)-ONO) are also formed. In contrast, when the analogous reactions were initiated with the weak sigma-donor ligand tetrahydrofuran or dimethyl sulfide present in the layers, formation of Mn(Por)(eta(2)-ONO2) is strongly favored (similar to 90%). The latter are formed via a 6-coordinate intermediate (L)Mn(Por)(eta(1)-ONO2) (L = THE or DMS) that loses L upon warming. These reaction patterns are compared to those observed previously with analogous iron and cobalt porphyrin complexes.