The intercalation of a typical UV-curable epoxy monomer (CE) in unmodified montmorillonite and the effect of hydration on the intercalation reaction are studied. Montmorillonite in the sodium form was submitted to a controlled hydration/dehydration cycle and the water content was checked by TGA/XRD analyses. The structure of the hydrated Na+-montmorillonite was determined from the values of the basal spacings and from the water content of the hydrated form: a coordination of four water molecules per Na ion was found, corresponding to a minimum of energy calculated by molecular dynamics simulation. When dispersing the clay in the CE monomer, the anhydrous Na+-montmorillonite did not show any intercalation; on the contrary the hydrated form showed an increase of the basal spacing. A possible mechanism to explain the intercalation of the CE monomers is proposed. (c) 2005 Elsevier Inc. All rights reserved.