To synthesize an epoxy-montmorillonite nanocomposite, the intercalation of clay layers with acid onium ions {[H3N(CH2)(n-1)COOH](+), [H3N(CH2)(n-1)CH3](+), [H3N(CH2)-NH2](+), and [H3N(CH2)(n)NH3](2+)) by different methods were studied. The intercalated clay was characterized by XRD, ICP, and CHN elemental analysis. An acid onium ion [H3N(CH2)(17)CH3](+) enlarged the d-spacing of CWC-, AMS-, and Kunipia-montmorillonite to 21.6, 18.1, and 20.7 Angstrom, respectively. The results of CHN elemental analysis confirmed that onium. ions were intercalated between the clay layers. Ion exchange of montmorillonite with onium ions is a better way to intercalate more onium. ions. Among those onium ions mentioned above, [H3N(CH2)(n)NH3](2+) displaced more sodium ions from the clay layers than did the others. An epoxy-montmorillonite nanocomposite was synthesized by heating a mixture of [H3N(CH2)(17)CH3](+)-montmorillonite (CWC) with an epoxy monomer and curing agent. The nanocomposite was characterized by XRD, TEM, DSC, UV, TGA, and Instron testing. TEM photographs showed that the spacing between the clay layers was further enlarged to about 50 Angstrom and no segregation between the clay particle and the polymer was observed. The glass transition temperature of the nanocomposite containing 20 phr of [HN(CH2)(17)CH3](+)-montmorillonite (CWC) increased from 108.4 to 146.1degreesC. Especially, the nanocomposite possesses higher water resistance than that of the epoxy resin or of the physical mixture of epoxy and montmorillonite (CWC). According to the transmittance measurement (lambda = 550 nm), the nanocomposite shows good light transmittance. Although, we cannot improve the mechanical properties, improvements of other properties can be evidenced for the formation of nanoscale composites.