SERS spectra of phenazine adsorbed on silver colloids have been investigated. For ligand concentrations from 10(-4) to 10(-6) M, it was possible to obtain the adsorption isotherm of phenazine by absorption measurements in the UV-visible region. Filtration of the ligand/hydrosol systems through suitable filters containing aluminum oxide membranes allowed a complete removal of the metal particles from the filtrate. Determination by spectral analysis of phenazine still dissolved in solution provided a quantitative estimate of the ligand adsorbed on the metal particles at different analytical concentrations. Colloids activated by chloride anions were used in these experiments to improve properly the Raman signal of the adsorbate. The results indicate that the SERS enhancement exhibits factors up to 10(6), in agreement with the previous theoretical and experimental findings.