The electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on two typical carbon electrodes, a glassy carbon electrode and a porous carbon electrode made of Ketjenblack, have been systematically evaluated in a DMSO-based electrolyte solution to develop a lithium-oxygen (Li-O2) battery with a high specific energy. The galvanostatic cycling was employed to characterize the electrochemical properties on these carbon electrodes during the ORR and OER processes. The ex situ far/mid-infrared spectroscopy and in situ UV-vis absorption spectroscopy were employed to quantitatively analyze the reaction products adsorbed on the electrode surface and dissolved in solution, respectively. Our results demonstrated that lithium peroxide (Li2O2) is a major ORR product on the porous carbon electrode, while the lithium superoxide (LiO2) becomes a major ORR product on a glassy carbon electrode. On the contrary, a large amount of byproducts, such as lithium carbonate (Li2CO3), was also observed only on the porous carbon electrode surfaces at the end of the ORR. Li2CO3 gradually decomposed with the increasing potential but remained on the electrode surface, depending on the upper potential limit of the OER. The accumulation of the byproducts during the ORR/OER results in electrode passivation and capacity fading. A better understanding of the ORR/OER mechanism and possible reason for poor reversibility on these electrodes has been achieved in this study.