A detailed experimental and theoretical vibrational analysis of arsenicin A, the first polyarsenical isolated from Nature, is reported. By exploiting the dissimilar and complementary selection rules of infrared (IR) absorption and Raman scattering transitions, and by carrying out density functional theory (DFT) calculations on several candidate structures, we are able to determine the structure of arsenicin A. We demonstrate that vibrational spectroscopy can be a very useful tool in structure elucidation in cases where mass spectrometry (MS) measurements lead to unreliable information, and nuclear magnetic resonance (NMR) investigations are hindered by the presence of silent nuclei, and/or elements of symmetry, in the molecule. Our approach allows to establish a reliable methodology, based on molecular vibrational parameters, for the structural elucidation of any unknown organic compounds of medium-small size on which, nowadays, accurate theoretical calculations can be carried out in reasonable times, without resorting to large-dimension computing resources.