Al-27 3QMAS 2D NMR was used to study the molecular-level organization of a clay pillared with the "Al-13-(heidi)(6)(3+)" polycation, whose unusual structure is stabilized by hydroxyethyliminodiacetate (heidi) ligands. The quadrupolar line shape and the isotropic chemical shifts were first unambiguously determined on a solid salt of the polycation. This compound was then used to identify the finger-print of the Al-13(heidi)(x)(3+) polycation in a sample of saponite clay that had been intercalated with a solution containing Al(III) and heidi ligands, showing that the basic structure of the Al polymer had been preserved upon intercalation. A more detailed analysis of 3QMAS 2D NMR line shapes indicated. however, that a selective partial hydrolysis reaction had occurred, most probably leading to Al-13(heidi)(3)(3+) polycations (loss of three heidi ligands). Thus, 3QMAS NMR allowed to choose one structural model for these nanocomposites from the several compatible with previously published characterizations.