The scattering spectra of single gold, silver, and bimetallic gold-silver particles (both core-shell and alloyed) have been examined using dark-field microscopy. The results show that the plasmon resonances for the bimetallic particles are broader than those of the pure silver or pure gold particles. However, plots of the width of the plasmon resonance vs resonance frequency for the core-shell and alloyed samples are very similar. This implies that the broadening is due to the frequency dependence of the dielectric constants of the particles. For the core-shell particles, scattering at the interface between the two metals does not seem to be a significant effect.