A new CE method with ultraviolet-visible detection was developed in this study to investigate manganese dissolution in lithium ion battery electrolytes. The aqueous running buffer based on diphosphate showed excellent stabilization of labile Mn3+, even under electrophoretic conditions. The method was optimized regarding the concentration of diphosphate and modifier to obtain suitable signals for quantification. Additionally, the finally obtained method was applied on carbonate-based electrolytes samples. Dissolution experiments of the cathode material LiNi0.5Mn1.5O4 (lithium nickel manganese oxide [LNMO]) in aqueous diphosphate buffer at defined pH were performed to investigate the effect of a transition metal-ion-scavenger on the oxidation state of dissolved manganese. Quantification of both Mn species revealed the formation of mainly Mn3+, which can be attributed to a comproportionation reaction of dissolved and complexed Mn2+ with Mn4+ at the surface of the LNMO structure. It was also shown that the formation of Mn3+ increased with lower pH. In contrast, dissolution experiments of LNMO in carbonate-based electrolytes containing LIPF6 showed only dissolution of Mn2+.