Low dielectric constant (low-k) nanocomposite thin films have been prepared by spin coating and thermal cure of solution mixtures of one of two organic low-k thermoset prepolymers and a silica nanoparticle with an average diameter of about 8 nm. The electrical, the mechanical, and the thermomechanical properties of these low-h nanocomposite thin films have been characterized with 4-point probe electrical measurements, nanoindentation measurements with an atomic force microscope, and specular X-ray reflectivity. Addition of the silica nanoparticle to the low-k organic thermosets enhances both the modulus and the hardness and reduces the coefficient of thermal expansion of the resultant nanocomposite thin films. The enhancements in the modulus of the nanocomposite thin films are less than those predicted by the Halpin-Tsai equations, presumably due to the relatively poor interfacial adhesion and/or the aggregation of the hydrophilic silica nanoparticles in the hydrophobic organic thermoset matrices. The addition of the silica nanoparticle to the low-k organic thermoset matrices increases the relative dielectric constant of the resultant nanocomposite thin films. The relative dielectric constant of the nanocomposite thin films has been found to agree fairly we 11 with an additive formula based on the Debye equation. (C) 2007 Wiley Periodicals, Inc.