Nearly monodispersed Au-12 clusters protected by monolayers of meso-2,3-dimercaptosuccinic acid (DMSA) exhibited luminescence at 630 nm with a quantum yield of 1 X 10(-6) upon the photoexcitation at 395 nm to the lowest electronic excited state. The visible PL is assigned to phosphorescence originating from a triplet-like excited state based on the large Stokes shift (1.2 eV). The PL quantum yield was enhanced up to approximate to0.9%, greater by eight orders of magnitude than that of bulk gold, by thickening the protecting layer and freezing a solvent at 77 K. The emission peak energy is discussed within the context of core-size dependent electronic structures by comparison with those of thiolate-protected gold clusters reported in the literature. (C) 2003 Elsevier B.V. All rights reserved.