A series of composite materials based on a hydrophobic mesoporous silica (MCM-41-m) filled polytetrafloroethylene (PTFE) were synthesized. The composite boards prepared from the composite materials were thermally stable (T-5% > 500 degrees C). The dielectric constant (D-k) and z-axis coefficient of thermal expansion (CTEz) were significantly decreased with increasing MCM-41-m content due to the presence of the well-dispersed hydrophobic and ordered mesoporous structure MCM-41-m particles. Ultra low D-k (1.70) and CTEz (11.8 ppm/degrees C) were obtained for the composite board containing 40 wt % of MCM-41-m. The result indicated that the as-prepared composite materials are potentially useful as radio-frequency/microwave substrate materials. (C) 2004 The Electrochemical Society.