This work seeks to characterize the influence of chloride on the surface-enhanced Raman scattering (SERS), optical absorption and fluorescence of methyl orange adsorbed on Ag colloids. The relation between SERS, absorption and fluorescence signals points out the importance of a first-layer effect to account for the changes in intensity and in the spectral profile observed when adding Cl-. A differing response of the Ag colloids to the excitation wavelength in presence or absence of Cl- indicates the influence of the halide on the morphology of the metal particles and subsequently the surface-coverage of adsorbate. The differences observed in the profiles of first-layer SERS spectra and the multilayer spectra may be attributed to a considerable contribution of the charge-transfer mechanism to the SERS effect, which is more efficient in the first-layer. (c) 2006 Elsevier B.V. All rights reserved.