Gaining insight into the reaction mechanisms underway during charge and discharge in Li-air batteries is essential to allow the target development of improved power and performance devices. This work reports the in situ monitoring of Li-air cells by electrochemical impedance spectroscopy and, for the first time, the development of an electrochemical model allowing the identification and attribution of the processes involved. The voltage at which each reaction product forms has been identified, including Li2O2 or Li2CO3 during discharge, together with the delithiation of the outer part of Li2O2 and oxidation reactions and electrolyte decomposition. The developed model can be used as a valuable tool for the optimisation of composition and structure of the air electrode through the investigation of the resistance associated with each process. (C) 2013 Elsevier B.V. All rights reserved.