Reinforcement with nanotubes, nanofibers, and nanoparticles is an attractive option for enhancing the properties of micromachined polymeric structures employed in microelectromechanical systems (MEMS). Calculations based on Eshelby-Mori-Tanaka micromechanics predict that the elastic modulus and wave velocity can be increased by over an order of magnitude by reinforcing polymers with aligned, dispersed, single-walled carbon nanotubes (SWNT). Motivated by this prediction, we measured the elastic moduli of polyimide films reinforced with SWNT at volume fractions ranging from 0 to 10%. For dilute composites, the elastic modulus increased with increasing nanotube loading from 2.5 GPa for the neat polymer to 3.5 GPa for a nanocomposite containing 0.5 vol% of SWNT. However, with further increase in the nanotube content, the elastic modulus remained essentially constant even for high loadings of 10 vol% of SWNT. In addition, significantly different elastic moduli were measured for specimens containing the same volume fraction (0.5 vol%) of SWNT produced by two different processes.