Discharging of the aprotic Li-O2 battery relies on the O2 reduction reaction (ORR) forming solid Li2 O2 in the positive electrode, which is often characterized by a sharp voltage drop (that is, sudden death) at the end of discharge, delivering a capacity far below its theoretical promise. Toward unlocking the energy capabilities of Li-O2 batteries, it is crucial to have a fundamental understanding of the origin of sudden death in terms of reactive sites and transport limitations. Herein, a mechanistic study is presented on a model system of Au|Li2 O2 |Li(+) electrolyte, in which the Au electrode was passivated with a thin Li2 O2 film by discharging to the state of sudden death. Direct conductivity measurement of the Li2 O2 film and in situ spectroscopic study of ORR using (18) O2 for passivation and (16) O2 for further discharging provide compelling evidence that ORR (and O2 evolution reaction as well) occurs at the buried interface of Au|Li2 O2 and is limited by electron instead of Li(+) and O2 transport.