The interfacial reactions, especially the gas evolution, between carbon conductive agents and the electrolyte at the positive electrode in high-voltage batteries (potentials over 4.5 V) have been investigated. The amount of gas generated was quantified for various conductive agents: acetylene black (AB), furnace black, specially customized AB, and graphite (GR). The experiments revealed that in the high-voltage system, the specific gas evolution was induced by both the cathode active material and the conductive agent, with the carbon conductive agents resulting in the generation of 8 to 15 times more gas than the cathode active material LiNi0.5Mn1.5O4 (LNMO) itself. The high-voltage properties of the carbon conductive agents, such as the anion intercalation and self-discharging properties, were evaluated for each carbon electrode. The results implied the existence of a local battery composed of the conductive agent and LNMO; this redox couple appears to play a key role in the gas evolution. (C) 2015 The Electrochemical Society. All rights reserved.