We present a high-resolution C K- and O K-NEXAFS study for the large aromatic molecule acenaphthenequinone (ANQ) in the condensed phase. This is the largest molecule for which a distinct vibronic fine structure at the 0 K-edge has been resolved so far. The most prominent results are the appearance of only one predominant vibrational mode, a C-H stretching mode, in the O 1 s NEXAFS spectra and the analysis of the anharmonicity of the excited state potential. (C) 2004 Elsevier B.V. All rights reserved.