Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been used to investigate the adsorption of crystal violet (CV + ) to a charged silica-water interface as a function of bulk pH by the direct measurement of the absorbance of the CV + chromophore. Absolute absorbances of order 10 -4 have been routinely detected, showing significant variation in the structure of the silica-water interface. At low ionic strength, the interfacial absorbance of CV + shows a monotonic increase with increasing pH. A simple competitive Langmuir adsorption model, which provides values for the silica surface parameters that are in broad agreement with the existing literature values, has been fit to the data. In addition, interfacial absorbance has been monitored as a function of pH for CV + solutions maintained at high ionic strength with NaCl, KCl, and CaCl 2 . As pH increases, the CV + interfacial absorbance exhibits a pronounced maximum, which occurs at pH 8.7 for Na + and K + and at pH 7.9 for Ca 2+ , followed by a sharp decrease. This trend is attributed to competitive binding between the metal cations and CV + to the silica surface binding site, and it has not been observed in previous measurements using second-harmonic generation. The simple Langmuir model, however, does not accurately describe the high ionic strength behavior.