A major challenge in the development of Li–O2 batteries is the high overpotential required for battery operation, especially during the charging process. Various precious metals and their corresponding oxides have been proposed as electrocatalysts to reduce the charge voltage of Li–O2 batteries. However, the real mechanism by which these solid catalysts decompose Li2O2 during the charge process is still under debate. Here, Ru nanoparticles supported on carbon matrixes with ultrahigh dispersibility were synthesized and evaluated as cathode materials for Li–O2 batteries. The charge voltage of the corresponding Li–O2 battery was reduced to below 4.1 V (V vs. Li/Li+), and a stable cycling performance over more than 100 cycles was achieved. Nevertheless, differential electrochemical mass spectrometry measurements revealed that large amounts of gaseous by-products evolved even during the first charge process, suggesting that severe parasitic reactions accompanied the decomposition of Li2O2. The undesired degradation of the carbon cathode and/or electrolyte was also accelerated in the presence of Ru. Thus, this study has revealed that previous conclusions regarding the role of Ru in catalytically decomposing Li2O2 may be misleading. Furthermore, the necessity of performing quantitative analysis of gas evolution in order to study the real performance of Li–O2 batteries has been demonstrated.