Ultrafast laser spectroscopy has been used to measure the electron-phonon coupling constant in PtAu core-shell bimetallic nanoparticles. A chemical reduction method was used to prepare Pt cores of 12.5 nm diameter and a gamma -radiolytic deposition technique was then used to grow Au shells of variable thickness. The resulting nanoparticles have a spectrum that is characteristic of Au. It is found that the electron-phonon coupling time for these hybrid nanoparticles (tau (e-ph) similar to 200 fs) is over a factor of 3 shorter than that for plain Au nanoparticles (tau (e-ph) similar to 650 fs). The faster dynamical response is due to the large electron-phonon coupling constant for Pt, which provides efficient scattering of the excited electrons. Platinum dominates the temporal response, even for a 1:6 molar ratio of Pt to Au, because it has a much larger density of states near the Fermi level compared to Au. (C) 2001 American Institute of Physics.