An electrochemical model was developed to describe alternating current (ac) impedance experimental studies conducted on lithium-ion positive electrodes. The model includes differential mass and current balances for the positive electrode's composite structure, as well as details of the oxide-electrolyte interface. A number of specialized experiments were conducted to help define the parameter set for the model. The electrochemical ac impedance model was used to examine aging effects associated with the positive electrode. (c) 2005 The Electrochemical Society. All rights reserved.