The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20nm mean diameter) in an aqueous solution has been investigated. Size dependent alloying was observed upon deposition of Ag on Au core. Ag forms distinct layers around the 20 nm Au nanoparticles. In contrast, random mixing is found for Ag deposited on 3.5nm Au particles.