Abstract: Cu montmorillonite heated with or without potassium halide was studied by IR and ESR spectroscopy, supplemented by XRD measurements, microprobe and chemical analyses. It appears that on heating Cu montmorillonite, most of the Cu ions migrate into hexagonal cavities and eventually, when dehydroxylation occurs, into octahedral vacancies. Some Cu ions may penetrate into octahedral vacancies before dehydroxylation. In the presence of potassium halide, deprotonation facilitates penetration of Cu into octahedral vacancies. The presence within the layers of non-exchangeable Cu ions that are inaccessible to water does not necessarily cause the perturbation of OH bending vibrations conventionally attributed to migration of small cations into the structure. Such perturbation was only observed when the basal spacing was reduced to ∼9.5Å.