Active lithium in the negative electrode of a Li-ion cell reacts with electrolyte to form an ever-thickening solid electrolyte interphase. The rate of this reaction can be monitored as a function of temperature, time and electrode potential using storage and symmetric cell studies. Using the soft carbon, petroleum coke, as a model negative electrode material, experiments measuring the open circuit voltage (OCV) change with time of Li/coke cells were made to measure the rate of loss of active lithium. The capacity loss with cycle number or time of coke/coke symmetric cells was also used to measure the rate of Li loss. The results on over 100 test cells show that: 1) the reaction rate decreases by about a factor of 2-4 as the electrode potential increases from 0.005 to 1.0 V; 2) the reaction rate increases approximately 3-10 fold between 30 and 60 degrees C depending on the electrode potential; 3) The reaction rates are within a factor of two, and may be the same, for electrodes at OCV or undergoing cycling; 4) the reaction rate is larger when vinylene carbonate (VC) is present in the electrolyte at 30 degrees C for all potentials and times studied and 5) the reaction rate is about two times smaller in the presence of VC at 60 degrees C for potentials above 0.4 V. A significant number of further experiments are required to develop accurate theoretical models of the reactivity of intercalated lithium with electrolyte as a function of time, temperature and potential. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.048210jes] All rights reserved.