A series of biscobalt cofacial porphyrin-corrole dyads bearing mesityl substituents at the meso positions of the corrole ring were investigated as to their electrochemistry, spectroelectrochemistry, and CO binding properties in nonaqueous media and then applied to the surface of a graphite electrode and tested as electrocatalysts for the reduction of dioxygen to water or hydrogen peroxide in air-saturated aqueous solutions containing 1 M HClO 4. The catalytic reduction of O 2 with the same dyads was also investigated in the homogeneous phase using 1,1′-dimethylferrocene as a reductant in PhCN containing HClO 4. The examined compounds are represented as (PMes 2CY)Co 2, where P = a porphyrin dianion, Mes 2C = a corrole trianion with two mesityl groups in trans meso-positions of the macrocycle, and Y is one of three bridging groups separating the two metallomacrocycles in a face-to-face arrangement, either with 9,9-dimethylxanthene, dibenzofuran, or diphenylether as linkers. Cyclic voltammetry and rotating disk electrode voltammetry revealed that the examined compounds are all catalytically active toward the electroreduction of dioxygen in acid media giving H 2O 2 or H 2O depending upon the type of linkage (Y) and the initial site of electron transfer which, in nonaqueous media, could be switched between the corrole and the porphyrin metal center by variations of substituents on the corrole macrocycle or the gas above the solution. The homogeneous reduction of dioxygen via a two- or four-electron transfer process was also investigated using 1,1′-dimethylferrocene as reductant in PhCN containing HClO 4.