Aiming at a better molecular scale understanding of electrolyte degradation processes at electrode|electrolyte interfaces typical for modern batteries, in particular for lithium ion batteries (LIBs), we have developed a novel Differential Electrochemical Mass Spectrometry (DEMS) approach, and applied this for analyzing the evolution of volatile decomposition products at a high voltage LiNi0.5Mn1.5O4 (LNMO) cathode. Using a standard LP30 lithium ion battery electrolyte as well as 1.0 M LiPF6 in either ethylene carbonate (EC) or dimethyl carbonate (DMC), respectively, the gas evolution rates during potentiodynamic cycling and in potential step experiments were monitored online, with high time resolution, in half-cell measurements. Following the potential and time dependent appearance of the reaction products and their fragments, the major gases evolved were identified as H2, CO2 and CO. Their formation at the LNMO electrode at high potentials is explained by Ni4+ catalyzed dehydrogenation of organic carbonates, as evidenced in the potential step experiments from the correlation of the charge passed and the continuous gas formation even after decay of current down to zero. Differences in the potential dependent and in the time dependent product formation rates in the three electrolytes point to a non-additive behavior of the solvents EC and DMC in LP30. The results clearly illustrate the potential of this set-up for detailed studies of electrolyte degradation processes in modern battery systems.