Conventional composites filled with clay as well as intercalated nanocomposites, and exfoliated nanocomposites based on a glassy epoxy matrix have been synthesised. Flexural moduli of these materials were measured in three-point bending at various clay contents. For a given clay content, stiffness improvements depended not only on the dispersion of the clay on the microscale, but also on the exfoliation of the clay layers at the nanolevel. Dynamic mechanical measurements indicated a decrease of intensity in the glass transition peak with the extent of exfoliation of the clay and the clay content, suggesting a restriction of the molecular mobility of the polymer in the vicinity of the clay layers. A shift in Tg of 20 deg C towards lower temperature for the epoxy resin cured at 160 deg C was possibly caused by thermal degradation of compatibilizing agents at high temperature.