A new method to stabilize silver nanoparticles by the addition of ammonia is proposed. Colloidal dispersions of silver nanoparticles were synthesized by the Turkevich method using sodium citrate to reduce silver nitrate at high pH and at 90 °C. After approximately 12 min, a diluted ammonia solution was added to the reaction flask to form soluble diamine silver (I) complexes that played an important growth moderating role, making it possible to stabilize metallic silver nanoparticles with sizes as small as 1.6 nm after 17 min of reaction. Colloidal dispersions were characterized by UV-visible absorption spectroscopy, X-ray diffraction, and transmission electronic microscopy.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CMDMC/Cepid-INCTMNUniversidade Estadual Paulista Júlio de Mesquita Filho, Departamento de Bioquímica e Tecnologia Química, Instituto de Química de Araraquara, Araraquara, Rua Francisco Degni, 55, Quitandinha, CEP 14801907, SP, BrasilUniversidade Estadual Paulista Júlio de Mesquita Filho, Departamento de Bioquímica e Tecnologia Química, Instituto de Química de Araraquara, Araraquara, Rua Francisco Degni, 55, Quitandinha, CEP 14801907, SP, BrasilFAPESP: 2007/58891-7