Gold nanoparticles (AuNP) can be used as seeds for the synthesis of larger AuNP of controllable size with narrow size distribution by photochemical reduction of additional Au(III) using water-soluble benzoins or H2O2 as sources of reducing radicals. Further, beyond simply enlarging the AuNP, it is possible to add a shell of another metal, such as silver, leading to Au/Ag core-shell structures with controllable dimensions for both core and shell. This strategy illustrates the fine spatial and temporal control achievable using clean photochemical techniques without the addition of hard surface ligands often necessary to control the size and structure of gold-silver nanostructures. The mild nature of the surface coverage makes these nanomaterials ideal for further surface modification.