This paper presents results (structural parameters and anharmonic vibrational spectra) obtained for the methylsodium system at different levels of theory from DFT and post-HF [CCSD(T)] calculations. These theoretical results are faced with the existing experimental data. An interpretation of the experimental reference is proposed from our calculations about the monomer and the dimer compounds. (c) 2006 Elsevier B.V. All rights reserved.