Identification of surface species by IR spectroscopy is often based on common knowledge, with a clear risk of misassignment. Combining density functional theory (DFT) and IR reflection absorption spectroscopy (IRAS), we investigated surface nitrate formation on BaO/Al2O3 thin films. We found that it is not possible to identify coordination geometries of nitrates based solely on their vibrational frequencies. For metal-supported thin films, however, differences in polarization of the NO stretching modes facilitate unambiguous identification. In contrast to traditional assignments, we show that both bridging and monodentate nitrates are formed on BaO, whereas on Al2O3 bridging nitrates dominate. (c) 2008 Elsevier Inc. All rights reserved.