Quantitative differential electrochemical mass spectrometry and cyclic voltammetry have been combined to probe possible mechanisms and the kinetic overpotentials, responsible for discharge and charge in a Li-O2 battery, using C as the cathode and an electrolyte based on dimethoxyethane as the solvent. Previous spectroscopy experiments (X-ray diffraction, μRaman, IR, XPS) have shown that Li2O2 is the principle product formed during Li-O2 discharge using this electrolyte/cathode combination. At all discharge potentials and charge potentials <4.0 V, the observed electrochemistry is ∼2e-/O2 consumed or produced, also implying that Li2O2 is the dominant thermodynamically stable species formed and consumed in the electrochemistry. No evidence exists at any potential for formation of stable LiO2 (1e-/O2) or Li2O (4e-/O2) during discharge. At charging potentials >4.0 V, the electrochemistry requires significantly more than 2e-/O2, and we take this as evidence for electrolyte decomposition. We find that sequential concerted (Li+ + e-) ion transfers to/from adsorbed O 2* and LiO2* to produce/consume Li 2O2 is the mechanism that is most compatible with these experiments. The kinetic overpotentials are extremely low relative to aqueous O2 reduction and evolution, and this implies that in principle a discharge-charge Li-O2 cycle is possible with high voltaic efficiency (∼85%) if electrolyte and cathode stability issues are resolved. © 2012 American Chemical Society.