The high flammability and thermal instability of conventional carbonate electrolytes limit the safety and performance of lithium-ion batteries (LIBs) and other electrochemical energy storage devices. Organosilicon solvents have shown promise due to their reduced flammability and greater chemical stability at high temperatures. A series of organosilicon electrolytes with different functional substituents were studied to understand the structural origins of this enhanced stability. The thermal and hydrolytic stability of organosilicon and carbonate solvents with LiPF6 was probed by storage at high temperatures and with added water. Quantitative monitoring of organosilicon and carbonate electrolyte decomposition products over time using NMR spectroscopy revealed mechanisms of degradation and led to the discovery of a key PF5-complex that forms in organosilicon electrolytes to inhibit further salt breakdown. Increased knowledge of specific structural contributions to electrolyte stability informs the development of future electrolyte solvents to enable the safer operation of high-performing lithium-ion batteries. (C) 2017 The Electrochemical Society. All rights reserved.