Studies using ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) demonstrate that M4X6 (M = W, Mo and X = O, S) clusters show large gaps (about 2 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), indicative of their high stability and chemical inertness. In particular, W4O6 has a lower symmetry and a larger HOMO-LUMO gap than other hitherto discovered magic clusters. Although the similarity between the electronic structures of W4O6 and Mo4S6 may be regarded as an indication that both clusters have similar geometric structures. our detailed DFT-calculations reveal otherwise. This result implies that synergetic approach using theoretical and experimental methods are essential to shed light on cluster geometries. (C) 2004 Elsevier B.V. All rights reserved.