An examination of the “hydroxyl region” of the spectra of brucite, gibbsite, kaolin family minerals, and montmorillonites in various states of dehydration show the spectra to be much more complex than at first expected. The presence of multiple absorption bands even in powder spectra of minerals such as brucite, gibbsite, and the clay minerals, cannot yet be explained in terms of the X-ray structure. There is no simple correlation of OH − absorption frequencies with “types of OH −,” and extent of hydrogen bonding, based on the data obtained. OD − is shown to replace OH − approximately equally in the three maxima in kaolinite and dickite in the 2·7micron region, when the minerals are reacted with D 2O at about 370°C and 20,000 p.s.i. The more common generalization that hydrogenbonded OH − ions show a bathochromic shift compared to the “free” or “symmetrical” OH − is also not borne out by our data, which show that, at least in part, the longer wavelength 2·9-μ maximum is the result of hydroxyls which remain in the structure of montmorillonite heated at elevated temperatures. Deuterium-exchange experiments indicate that the exchange of hydroxyls varies widely with temperature. Attempts of 1-month duration to exchange OD − for OH − at room temperature indicated practically no exchange in dickite and kaolinite, with only a small amount in montmorillonite. Under elevated pressures, a significant amount of exchange was observed in kaolinite at 190°C, and extensive exchange took place in kaolinite at 370°C (greater than 50%), somewhat less in montmorillonite, and the least in dickite. In all the OD − phases and in synthetic Mg(OD) 2, the OD − absorption maxima are moved to greater wavelengths by a multiple of about 1·37 from the corresponding OH − maxima in the original minerals.