Infrared spectroscopy has been used in adsorption studies of trimethyl phosphite (TMP) on the hydrogen (H) form of the A-type and 13X zeolites. Infrared spectra of H-zeolites, except H-KA, show that TMP reacts rapidly with silanol hydroxyls on their surfaces and gives rise to form product molecules such as SiOCH3 and liquid dimethyl phosphite. The unaccomplished adsorption on the H-KA zeolite has been attributed to the occurence of stable KOH hydroxyls with not-exchanged K cations instead of SiOH hydroxyls on this zeolite.