We report a one-step route for the synthesis of highly luminescent and water-soluble Au 18SG 14 (SG- glutathione in thiolate form) in nearly pure form using a slow reduction process. The cluster shows step-like behavior in its absorption profile. It emits red light in both aqueous and solid state under UV illumination. Quantum yield of the cluster is 0.053, nearly 25-fold higher than that of Au 25SG 18. The cluster exhibits distinct features corresponding to multiply charged ions in electrospray ionization mass spectrometry. This composition is also confirmed from MALDI MS along with other quantitative analyses. The cluster makes closed shell molecular ions in the gas phase. The possibility of making clusters of different core sizes is also demonstrated. The simplicity of this method and identification of the cluster with exact composition may facilitate the exploration of experimental and theoretical research on this material. © 2012 American Chemical Society.