Monodisperse, bimetallic PtCo nanoparticles with a controllable core-shell structure have been synthesized by a new two-stage route. The process involves thermal decomposition of cobalt carbonyl in the presence of a nanosized platinum (Pt) dispersion, where the thickness of the Co shell can be controlled by changing the amount of cobalt carbonyl. The synthesized particles reveal a narrow size distribution with average diameters of 2.6 for Pt and a maximum of 7.6 nm for PtCo. Transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), optical spectroscopy, and superconducting quantum interference (SQUID) magnetometry were used for characterization of the bimetallic system.