In this study we report the strong effect of water or organic carbonates on the specific capacity during the first discharge of the cathode of a Li-O-2 cell, using Vulcan-based positive electrodes. An improved cell design allowed us to avoid contaminations from atmospheric air and to compare the discharge in water-free electrolyte with that of water contaminated cells, getting an increased specific capacity up to one order of magnitude. Understanding the mechanism that limits or enhances the specific capacity of the positive electrode is very important in order to improve performance and durability of Li-O-2 cells. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.005204esl] All rights reserved.