Ag-coated Au colloidal particles have been prepared by reduction of Ag+ in the presence of preformed Au colloids. The composition of the Au100-xAgx particles was varied from x = 0 to 80. SERS spectra of pyridine, p-nitroso-N,N’-dimethylaniline (p-NDMA), and trans-1,2-bis(4-pyridyl)ethylene (BPE) have been obtained with these colloids. At monolayer Ag coverages (x < 10), the optical spectra of Ag-coated Au particles are indistinguishable from uncoated Au particles. However, the SERS behavior of aggregated colloids with 647.1 nm excitation is extremely dependent upon the Ag:Au ratio. Very small amounts of Ag (x less than or equal to 5) lead to an increase in SERS intensity, but further increases lead to complete loss of signal. For p-NDMA and pyridine, these data can be explained by Ag inhibition of adsorbate-induced aggregation. The initial increase in SERS intensity results from production of smaller aggregates that exhibit a surface plasmon band in better alignment with the excitation wavelength; higher ratios of Ag eliminate aggregation and all SERS enhancement. For BPE, the same Ag-induced loss of SERS is observed, even though each of the Au100-xAgx colloidal solutions is clearly aggregated by BPE adsorption. This finding suggests that submonolayers of Ag modulate specific chemical interactions between the Au and BPE that are responsible for SERS.