The behavior of three gas diffusion electrodes (GDE) with macro- and mesopores is investigated by in situ electrochemical impedance spectroscopy (EIS) in the Li/O2 battery system while discharging. Using a three electrode setup the current response from the anode (Li metal) and cathode (GDE) can be separated and the changes of the electrochemical processes at the GDE during discharge can be observed, exclusively. We identify up to four processes with different time constants which we assign to the lithium ion migration through a surface layer, the charge-transfer from the carbon surface to the molecular oxygen, the lithium ion and oxygen diffusion towards the cathode surface and, in case of the mesoporous materials, the lithium ion movement inside the pores, along the pore axis. The latter finding reflects that pore clogging of such is a limiting factor for the discharge of the Li/O2 battery. A large mesopore volume as in the xerogel electrode, however, allows for a high storage capability and a long and constant oxygen reduction. We demonstrate that the three electrode EIS proves to be a powerful in situ diagnostic tool to determine the state and, hence, the reversibility of the reactions at the cathode.