Surfactant adsorption in porous silicon films formed by anodic oxidation of bulk p-type silicon in HF solution was studied by spectroscopic ellipsometry. The cationic surfactant cetyltrimethylammonium bromide reversibly adsorbed in the films from aqueous solutions. No adsorption of sodium dodecyl sulfate was observed. Modification of the surfaces by addition of aluminum nitrate from ethanol solutions and titanium(IV) chloride from the gas phase, followed by thermal decomposition, altered the surface propel ties and surfactant adsorption behavior. A simple two-layer optical model was used to obtain the effective refractive indices for the porous films in different ambient media. Spectra calculated from the model agreed well with the experimental observations.