The electroconductivity and microwave properties of high-density polyethylenes containing 0-20 mass% acethylene carbon black were investigated. A correlation between electroconductivity and ultra-high frequency (UHF) electromagnetic waves absorption for systems containing carbon black in concentrations lower than that at the percolation threshold was found. The increase of carbon black (CB) concentration above the percolation threshold influences electrical conductivity slightly, but considerably changes microwave behavior. The highest coefficient of attenuation was registered in the 2.0-3.0 GHz range