The electrochemical behavior of a lithium/sulfur (Li/S) battery was studied by electrochemical impedance spectroscopy (EIS). An impedance model based on the analysis of EIS spectra as a function of temperature and depth of discharge was developed. Then, by monitoring the evolution of impedance during the cycling process, the capacity fading mechanism of Li/S battery was investigated. The results show that the semicircle at the middle frequency of the EIS spectra is ascribed to the charge-transfer process and the semicircle at high frequency is related to the interphase contact resistance. Furthermore, electrolyte resistance, interphase contact resistance, and charge-transfer resistance vary with cycle number in different manners, and the charge-transfer resistance is the key factor contributing to the capacity fading of Li/S battery. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.026304jes] All rights reserved.