We present a comparative study of nonaqueousLi−O2 and Na−O2 batteries employing an ether-basedelectrolyte. The most intriguing difference between the twobatteries is their respective galvanostatic charging over-potentials: a Na−O2 battery exhibits a low overpotentialthroughout most of its charge, whereas a Li−O2 battery has alow initial overpotential that continuously increases to veryhigh voltages by the end of charge. However, we find that theinherent kinetic Li and Na−O2 overpotentials, as measured ona flat glassy carbon electrode in a bulk electrolysis cell, aresimilar. Measurement of each batteries’ desired product yield,YNaO2 and YLi2O2, during discharge and rechargeability by differential electrochemical mass spectrometry (DEMS) indicates that less chemical and electrochemical decomposition occurs in a Na−O2 battery during the first Galvanostatic discharge−charge cycle. We therefore postulate that reactivity differences (Li2O2 being more reactive than NaO2) between the major discharge products lead to the observed charge overpotential difference between each battery.