The oxygen reduction reaction (ORR) in DMSO containing LiPF6 has been studied with the electrochemical quartz crystal microbalance (EQCM) during galvanostatic cathodic and anodic pulses, chronoamperometry and cyclic voltammetry experiments. We disclose here for the first time gravimetric evidence with the EQCM of the different mechanistic models that apply to the ORR in LiPF6/DMSO electrolyte: (i) Surface electrochemical/chemical deposition of Li2O2(s) and (ii) solution phase disproportion of LiO2(soln) yields large Li2O2 particles. The mass per electron detected with the EQCM depends on the respective current rates and thus on the mechanisms, i.e. 23g/F for the formation of Li2O2 thin film at high current density and much larger values for low current density due to solvent co-deposition and further degradation.