Resonance Raman spectra have been recorded from Methyl Red, Ethyl Red, α-Naphthyl Red, o-aminoazotoluene and 4-(phenyl-azo)diphenylamine adsorbed from toluene solutions on to untreated and calcined silica–alumina. The spectra show clear evidence for the protonation of all five molecules, resulting from interaction with Brønsted acidic sites on the silica–alumina surface. Adsorption data over a wide range of equilibrium concentrations give satisfactory fits to Langmuir isotherms. The densities of acidic sites increase after calcination at 300°C.