The optical absorption properties of 5,10,15,20-tetrakis(4-chlorosulfonylphenyl)-porphyrin (S4-Cl) and its mono- and cis-di-substituted analogues (S1-Cl and S2-Cl, respectively) adsorbed from solution onto clean and amine-modified silica suggest that similar mechanisms are responsible for the adsorption onto the two different substrates. In that adsorption onto amine surfaces is effected by sulfonamide linkages formed from reaction of sulfonyl chloride with surface-bound primary amine, the optical absorption data suggest that an analogous reaction involving surface hydroxyls of the unmodified silica may lead to porphyrin immobilization via sulfonylester linkages to the silica substrate. Resonance Raman (RR) spectra obtained for S4-Cl, S2-Cl and S1-Cl adsorbed to unmodified silica show significant differences to those obtained for the solution-phase species, suggesting that RR spectra are sensitive to the chemistry of peripheral phenyl substituents. When compared with spectra of the analogous sulfonatophenylporphyrin derivatives in solution, the RR data imply that the sulfonyl chloride functionalities take on -SO3 character upon adsorption : Such character could arise via either hydrolysis accompanying a physisorption process or esterification accompanying the chemisorption reaction of the -SO2Cl with the surface hydroxyls. Further evidence is obtained from an X-ray photoelectron study of the adsorption of benzenesulfonyl chloride onto silica, which shows no evidence for such a reaction. Therefore, it appears as though the sulfonylchloride porphyrins adsorb to silica via a physisorption mechanism and that the RR data evince the effects of hydrolysis on the vibrational properties of the physisorbed porphyrin species.