The activity for the oxygen reduction reaction (ORR), which is the reaction at the air electrode of fuel cells, depends on the surface structure of the electrode markedly. This review summarizes the effects of hydrophobic species on the activity for the ORR on single crystal electrodes of Pt. We have examined the effects of aliphatic and aromatic molecules on the ORR. Structural effects on the increase of the activity and the factors of the enhancement are discussed.