Transition-metal phosphates (TMPs) are potential materials for large-scale applications of lithium ion batteries (LIBs). Yet, high-voltage TMP cathodes have not met commercial success due to ill understood failure mechanisms. In this article we studied the surface chemistry of Li3V2(PO4)3 composite electrodes using X-ray photoelectron spectroscopy (XPS) post-electrochemical cycling in a stable electrochemical window of 3.0-4.2 V vs Li/Li+ and in the wider window of 3.0-4.8 V vs Li/Li+ where a dramatic fade in capacity is noted. In addition, we performed aging experiments in LiPF6 EC/DEC electrolyte with no electrochemical bias applied to investigate a possible spontaneous solid electrolyte interphase (SEI) formation as has been described for lithium transition-metal oxide (LixMyOz) electrodes. An SEI was found on the Li3V2(PO4)3 composite electrodes cycled in both potential windows and after aging with similar chemical compositions including ethers, alkoxides, esters, carboxylates, and carbonates as well as decomposed salt products. Analogous experiments were performed on the individual constituents of the composite electrode (active material, binder, and carbon additive). It was determined that the carbon additive and not Li3V2(PO4)3 formed an SEI both spontaneously and electrochemically. Therefore, the carbon additive and its properties are crucial in the formation of the SEI on TMP cathodes for LIBs which directly affect its lithium intercalation performance.