Ag-Pt nanoparticles with a confirmed core-shefl structure could only be formed by the successive reduction method using Ag nanoparticles as the seeds. The core-shell structure could be conveniently inferred from the transferability of the particles from water to toluene. Independent measurements by UV-vis spectroscopy, transmission electron microscopy, energy-dispersive X-ray analysis, and X-ray photoelectron spectroscopy were used to validate the experimental results. The reverse order of synthesis using Pt nanoparticles as the seeds did not result in any core-shell product. Instead a physical mixture of Ag nanoparticles and the original Pt seeds was obtained under the same experimental conditions.