Copper metal clusters of 4–5 Å have been intercalated in the interlayers of montmorillonite by in situ reduction of bulky copper acetate hydroxide species with ethylene glycol (polyol process) at 195 °C. Various amounts of Cu^II as copper acetate hydroxide hydrate in excess of ion-exchange capacity were introduced into the interlayers by titrating copper acetate solution with sodium hydroxide in the presence of montmorillonite. The exchange of fully developed copper acetate hydroxide hydrate interlayers in the montmorillonite gave a basal spacing of 19.6 Å. The reduced samples exhibited a basal spacing of 14.4 Å indicating the presence of 4.8 Å copper metal clusters after subtracting the thickness of the silicate layer (9.6 Å). Further evidence for formation of copper metal clusters in the interlayers was obtained from transmission electron microscopy (TEM) and UV-VIS-NIR spectroscopy. Both water sorption isotherms and nitrogen B.E.T. surface-area measurements indicated the porous nature of Cu metal intercalated phases similar to pillared clays. During the process of reduction, large amounts of copper metal particles were expelled from the interlayers which grew to ca. 0.1-0.5 µm on the external surfaces of montmorillonite.