The adsorption and dissociation of (\mathrm{CH}{3} \mathrm{Cl}) and (\mathrm{CH}{3} \mathrm{I}) have been investigated by means of infrared spectroscopy at 193-573 K. Both compounds adsorb molecularly on (\mathrm{Pd} / \mathrm{SiO}{2}) at (193 \mathrm{~K}) and interact strongly with the (\mathrm{OH}) groups of silica to form (\mathrm{H})-bridge-bonded alkyl halides. This was exhibited by a significant attenuation of the absorption band at (3744 \mathrm{~cm}^{1}) and by the development of a broad spectral feature at (3607 \mathrm{~cm}^{-1}). The absorption band due to adsorbed (\mathrm{CH}{3}) species, indicating the dissociation of (\mathrm{CH}{3} \mathrm{Cl}), was detected first at (263 \mathrm{~K}) and was eliminated above (373 \mathrm{~K}). Preadsorbed (\mathrm{CO}) and (\mathrm{O}) exerted no observable influence on the adsorption and dissociation (\mathrm{CH}{3} \mathrm{Cl}). However, potassium additive promoted the dissociation and somewhat stabilized the (\mathrm{CH}{3}) group on the surface. The dissociation of (\mathrm{CH}{3} \mathrm{I}) occurred at lower temperature and to a greater extent than that of (\mathrm{CH}_{3} \mathrm{Cl}). 1993 Academic Press. Inc.