Intercalated compounds of montmorillonite and poly-6-amide were prepared by heating mixtures of ɛ-caprolactam and 5, 10, 15 and 30 wt% of montmorillonite, cation-exchanged with NH3+(CH2)11COOH, at 250°C in a nitrogen atmosphere, and examined by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), gel permeation chromatography (GPC) and elemental analysis. ɛ-caprolactam was polymerized to poly-6-amide by the heat treatment. The mean number-average molecular weight decreased from 3·6 × 104 to 0·8 × 104 with increasing montmorillonite content. TEM showed that considerable swelling had taken place subsequent to polymerization and that the clay layers were uniformly distributed in the final clay/poly-6-amide products. Both XRD and TEM showed that the basal spacing of the compound increased from 51 Å to 210 Å with increasing polyamide content. The driving force for the swelling was considered to be a combination of the polymerization energy of ɛ-caprolactam and the attractive interaction between the interlayer cation and ɛ-caprolactam.