We have measured the dephasing time T-2 of surface plasmon excitation in oblate gold nanoparticles on sapphire substrates as a function of the particle size and shape. The influence of inhomogeneous line broadening was eliminated by applying the technique of persistent spectral hole burning. The obtained dephasing times vary between 9 and 15 fs depending on the particle shape and size. For the investigated nanoparticles with equivalent radii between 6 and 13 nm, a considerable deviation from bulk behavior, i.e., a higher damping as an influence of the reduced dimension of the nanoparticles was observed. The deviation from bulk behavior is ascribed to surface scattering of the oscillating conduction electrons. (C) 2004 Elsevier B.V. All rights reserved.