We explore the chemical contribution to surface-enhanced Raman scattering (SERS) in mixed-metal substrates, both experimentally and by computer simulation. These substrates are composed of a chemically active, transition- metal overlayer deposited on an effective SERS substrate. We report improved analytical enhancement factors obtained by using a small surface coverage of palladium or platinum over nanostructured silver substrates. Theoretical predictions of the chemical contribution to the surface enhancement using density functional theory support the experimental results. In addition, these approaches show that the increased enhancement is due not only to an increase in surface coverage of the analyte but also to a higher Raman scattering cross section per molecule. The additional chemical enhancement in mixed-metal SERS substrates correlates with the binding energy of the analyte on the surface and includes both static and dynamical effects. SERS using mixed-metal substrates has the potential to improve sensing for a large group of analyte molecules and to aid the development of chemically specific SERS-based sensors.