Li(Ni0.4Co0.15Al0.05Mn0.4)O-2 was investigated to understand the effect of replacement of the cobalt by aluminum on the structural and electrochemical properties. In situ X-ray absorption spectroscopy (XAS) was performed, utilizing a novel in situ electrochemical cell, specifically designed for long-term X-ray experiments. The cell was cycled at a moderate rate through a typical Li-ion battery operating voltage range. (1.0-4.7 V) XAS measurements were performed at different states of charge (SOC) during cycling, at the Ni, Co, and the Mn edges, revealing details about the response of the cathode to Li insertion and extraction processes. The extended X-ray absorption fine structure (EXAFS) region of the spectra revealed the changes of bond distance and coordination number of Ni, Co, and Mn absorbers as a function of the SOC of the material. The oxidation states of the transition metals in the system are Ni2+, Co3+, and Mn4+ in the as-made material (fully discharged), while during charging the Ni2+ is oxidized to Ni4+ through an intermediate stage of Ni3+, Co3+ is oxidized toward Co4+, and Mn was found to be electrochemically inactive and remained as Mn4+. The EXAFS results during cycling show that the Ni-O changes the most, followed by Co-O, and Mn-O varies the least. These measurements on this cathode material confirmed that the material retains its symmetry and good structural short-range order leading to the superior cycling reported earlier. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3494211] All rights reserved.