The solid electrolyte interface (SEI) formed via electrolyte decomposition on the anode of lithium ion batteries is largely responsible for the stable cycling of conventional lithium ion batteries. Similarly, there is a lesser-known analogous layer on the cathode side of a lithium ion battery, termed the cathode electrolyte interface (CEI), whose composition and role are debated. To confirm the existence and composition of the CEI, desorption electrospray ionization mass spectrometry (DESI-MS) is applied to study common lithium ion battery cathodes. As a surface ionization method, DESI does not require matrix addition or high-energy radiation, reducing the perturbation of the surface. We observe CEI formation on both LiMn₂O₄ and LiCoO₂ after running half-cells using 1 M LiPF₆ or LiClO₄ in ethylene carbonate (EC) with dimethyl carbonate (DMC). Intact poly(ethylene glycol) dimethyl ether is identified as the electrolyte degradation product on the cathode surface by the high mass-resolution Orbitrap mass spectrometer after five cycles between 3.5 and 4.5 V vs. Li/Li⁺. When EC is paired with ethyl methyl carbonate (EMC), poly(ethylene glycol) dimethyl ether and poly(ethylene glycol) methyl ethyl ether are found on the surface simultaneously. The presence of ethoxy and methoxy end groups indicates both methoxide and ethoxide are produced and involved in the process of oligomerization. Au surfaces cycled under different electrochemical windows as model systems for Li-ion battery anodes are also examined. Interestingly, the identical oligomeric species to those found in the CEI are found on Au surfaces after running five cycles between 2.0 and 0.2 V vs. Li/Li⁺ in half-cells. These results show that DESI-MS provides intact molecular information on battery electrodes, enabling deeper understanding of the SEI or CEI composition.