We describe herein a synthetic technology for precise size control of monodispersed hybrid gold nanoparticles, which combines seeding growth and digestive ripening. In this procedure, alkyl amines allow the solubilization and thermal reduction of HAuCl(4) while 2.1 nm gold nanoparticles capped by thiol functions are used as seeds. By carefully controlling the HAuCl(4)/seeds ratio, highly monodisperse gold nanoparticles with various sizes (from 2.1 to 8.8 nm) can be generated. Variation of the alkyl amines as well as optimization of the growth procedure allows the precise tuning of particle size within a 0.5 nm range. On the basis of XPS and TEM studies, a mechanism for nanoparticle formation is proposed.