The rheology, dc-conductivity, and microwave properties of acrylic, polyurethane, and epoxy composites containing 0-15 vol % of nanosized carbon particles have been investigated. Carbon nanoparticles (1-3 nm) are produced by a shock wave technology. Steady-state shear and oscillatory flow tests are applied to investigate the rheological properties of dispersions; dc-conductivity and MW absorption/reflection are investigated for solid composite films. Rheological characteristics are used for the evaluation of agglomeration processes of nanoparticles in dispersions, as controlled by volume fraction and processing technology. The percolation threshold is interpreted as a structural transition from a dispersed to an agglomerated state and it is found to depend significantly on the type of the matrix polymer. Above the percolation threshold, the presence of carbon nanoparticles produces a strong increase in the viscosity of dispersions as well as of the electrical conductivity and microwave properties of solid composites. A good correlation between the three characteristics is found for the systems in a wide range of carbon volume fractions. (C) 2004 Wiley Periodicals, Inc.