The solvatochromic method has been used to probe the solid/solution interface of bare silica and two modified silica surfaces-phenyl bonded and C 18 bonded-in mobile-phase mixtures of methanol-water and acetonitrilewater. Spectral measurements of solvatochromic dye solutions in different mobile-phase compositions and of slurries wetted by these same solutions have been recorded and used to characterize the different solid/ solution interfaces. Multivariate curve resolution has been employed to resolve the spectra collected into the contributions due to the different solvated species of the dye, i.e., those related to the dye associated with the stationary phase and those related to the dye solvated by the different species present in the mobile phase. Spectral profiles of the dyes solvated by a methanol-water complex in the presence of stationary phase have been resolved for the first time. Chromatographic capacity factors (k') have been measured, and they have complementary information about how the retention of the dye changes with the mobile-phase composition on the different silica surfaces. The validity of the spectral studies performed to characterize real chromatographic environments, which are usually under much higher pressures, has been investigated. In a wide range going from atmospheric pressure to values higher than 100 bar, no significant variations of the capacity factors were observed for the dyes used, and therefore, the information about retention mechanisms and solid/solution interface properties obtained from the spectral studies can be safely extrapolated to the real chromatographic systems. The results obtained indicate that the phenyl bonded silica shows a dipolarity/ polarizability very similar to that of bare silica. For these two silica surfaces, the interactions of the dye and the stationary phase are independent of the mobile-phase composition. The C 18 bonded silica has a significantly lower polar character and evidences two retention mechanisms depending upon the mobile-phase composition.