The synthesis and characterization of a series of iron and cobalt "picnic basket" porphyrins are described. Dioxygen binding to iron and cobalt picnic basket porphyrins has been studied by H-1 NMR and oxygen affinity measurements. These iron and cobalt picnic basket porphyrins bind dioxygen reversibly at room temperature with high affinities. The oxygen affinity increases as the basket size decreases. This effect indicates that a dipole-dipole interaction between the terminally bound dioxygen and the amide protons may play a role in the stability of the oxygen adducts. H-1 NMR spectroscopy does not support the existence of a strong H-bond in these picnic basket porphyrins. Oxygen affinities of the cobalt picnic basket porphyrins are more sensitive to the change of basket sizes than are the corresponding iron porphyrins. This result is consistent with the idea that the (CoO2)-O-II adducts have more electron density on the oxygen ligand than do the (FeO2)-O-II adducts.