The adsorption and nucleation of gold clusters Aun (n = 1–5) on montmorillonite (MMT) is studied using the density functional theory. All the calculations were performed using the full-potential linearized augmented-plane-wave method as implemented in the WIEN2k code. We constructed a MMT supercell of formula Si16Al6Mg2O40(OH)8 containing four unit cells and the first moments of the nucleation process were studied by adding the gold atoms one by one on the MMT. The results show that the interaction energies between the gold clusters and the MMT are negative indicating that the Aun–MMT complexes are stable. In the Aun–MMT systems (n = 3–5), two gold atoms maintain the coordination with basal oxygens and the AuO distance was about 2.0–2.3 Å. The AuAu average distance is 2.6 Å in the supported gold clusters. The formation of the second layer of gold atoms occurs upon the arrival of the third gold atom. We are reporting that the transition of 2D → 3D gold cluster structures is located from Au3 to Au4 on MMT. The total density of states of clusters Au, Au2, Au3, Au4, and Au5 on MMT allows us to affirm that (i) gold atoms are the main contributors to the states closest to the Fermi level and (ii) in Au4–MMT and Au5–MMT systems, the main contributors to the states closest to the Fermi level are the most external gold atoms, and therefore these atoms are probably the most susceptible to interact with adsorbates and the most active sites. Odd–even oscillations in the values of the energy gap and interaction energy were found: the odd-numbered supported clusters, Au3 and Au5, have larger energy gaps and more negative interaction energies. © 2012 Wiley Periodicals, Inc.