The reductive decomposition mechanism of electrolyte solutions for lithium secondary batteries has been investigated by electron spin resonance (ESR) with the aid of molecular orbital (MO) calculations. Discussion is focused on the initial reactions with the ultimate goal of suppressing decomposition. Solvent-related radical species were observed by ESR measurements in all of the electrolyte solutions after reductive electrolysis. The MO calculations suggest that the electron transfer from the electrode to the solvent molecule coordinated with the lithium cation upon cathodic polarization of the electrode is thermodynamically favorable. It is concluded that this electron transfer is the initial process in the decomposition of electrolyte solutions by electrochemical reduction. The radical species observed by ESR is attributed to the radical anion of the solvent coordinated with the lithium cation. Furthermore, ESR analysis indicated that the solvent radical anion and the neighboring solvent molecules coordinated with the same lithium cation are in electron-transfer equilibrium. The subsequent decomposition is initiated from this equilibrium.