Résumé : Data have been obtained on exclusive single neutron knockout cross sections from 12Be to study its ground state structure. Preliminary cross sections for the first (0.32 MeV, 1/2-) and second (1.78 MeV, 5/2+, unbound) excited states in 11Be have been obtained, giving evidence of significant admixtures of both ν(1p1/2) 2 and ν(1d5/2)2 configurations in the ground state of 12Be. © Società Italiana di Fisica / Springer-Verlag 2005.