We performed a comprehensive gas-phase experimental and quantum-chemical study of the binding properties of molecular oxygen to iron and manganese porphyrin anions. Temperature-dependent ion-molecule reaction kinetics as probed in a Fourier-transform ion-cyclotron resonance mass spectrometer reveal that molecular oxygen is bound by, respectively, 40.8 +/- 1.4 and 67.4 +/- 2.2 kJ mol(-1) to the Fe-II or Mn-II centers of isolated tetra(4-sulfonatophenyl)metalloporphyrin tetraanions. In contrast, Fe-III and Mn-III trianion homologues were found to be much less reactive-indicating an upper bound to their dioxygen binding energies of 34 kJ mol(-1). We modeled the corresponding O-2 adsorbates at the density functional theory and CASPT2 levels. These quantum-chemical calculations verified the stronger O-2, binding on the Fe-II or Mn-II centers and suggested that anion. O-2 binds as a superoxide anion.