The synthesis and characterization of water-soluble nitric oxide (NO)-releasing monolayer-protected gold clusters (MPCs) are reported. Tiopronin-protected MPCs (similar to 3 nm) were functionalized with amine ligands and subsequently exposed to 5 atm of NO to form diazeniumdiolate NO donors covalently bound to the gold MPC. Diazeniumdiolate formation conditions, NO-release, and nanoparticle stability were examined as a function of the structure of the protecting ligand, pH, and storage time. Despite their aqueous solubility, proton-initiated decomposition of the diazeniumdiolate-modified Tio-MPCs resulted in only modest NO-release (< 0.023 mu mol/mg) for short durations (< 1.5 h). To increase the NO storage capacity of gold nanoparticles, polyamine-stabilized MPCs (similar to 5 nm) were synthesized with significantly enhanced NO-release properties (0.386 mu mol/mg) and durations (up to 16 h). Transmission electron microscopy, thermogravimetric analysis, nuclear magnetic resonance spectroscopy, elemental analysis, and UV-vis spectroscopy were used to characterize both nanoparticle systems before and after NO exposure. The MPCs represent the smallest water-soluble NO-release nanoparticles to date (3-5 nm).