Various triphormic dioctahedral minerals and their dehydroxylation and rehydroxylation products have been examined by chemical, thermal, X-ray and infra-red absorption techniques. Since minerals with high Al-for-Si substitution do not rehydroxylate readily under the mild conditions used, attention was focused largely upon pyrophyllite and montmorillonite. Rehydroxylation occurs rapidly at about 400–500°C. Infra-red spectra indicate that there is a structural resemblance between the layers of all the dehydroxylates and that the rehydroxylates possess regions similar to untreated pyrophyllite. X-ray results show that stacking of the layers cannot be identical in the various samples. Although the structures could not be determined, the scheme proposed by Jonas for the rehydroxylate could not be reconciled with experimental results: nor does rehydroxylated montmorillonite resemble the abnormal form.