The potential NOx-reduction electro catalyst lanthanum strontium cuprate has been characterized with an in situ X-ray spectrochemical gas reaction cell. In a series of samples with increasing substitution of trivalent lanthanum by divalent strontium ions, the oxidation state of copper increased, maintaining charge neutrality, with the concentration of oxygen vacancies likely increasing at substitution ratios larger than Sr/La > 0.08. During heating in air, the valence of copper ions in the structure increased. Upon exposure to NO at 500 degrees C the valence of copper ions in a lanthanum strontium cuprate sample decreased, whereas it remained unchanged in the strontium-free lanthanum cuprate sample. (C) 2010 Elsevier B.V. All rights reserved.