The adsorption of methyl halides on silica surfaces has been investigated by FT-IR spectroscopy. The relative intensities of the adsorbate vibrational bands in the 2800-3100-cm-1 region differ from those found in the gas phase. We have correlated the relative change in-vibrational band intensities to a decrease in Fermi-resonance interaction between nu1, the C-H symmetric stretch, and 2nu5, the overtone of the asymmetric deformation, upon adsorption. CH3Cl, CH3Br, and CH3I exhibit a greater than 25% change in relative intensities upon adsorption, whereas CH3F exhibits only a 7% change. This study illustrates that care must be taken when relying on comparisons with gas phase vibrational spectra for the analysis of adsorbed molecules.