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The results of an investigation of the broadband dielectric properties of onion-like carbon (OLC)/polyurethane (PU) composites over a wide temperature range (300–450 K) are presented. The onset of the percolation threshold reached at 10 vol% of OLC is accompanied by drastically increased microwave absorption. The broadband dielectric properties of the composites at low loading of OLC, below the percolation threshold, are mainly governed by the α relaxation, the dynamics of the “soft” molecular segment of the PU chain. With increased OLC concentration, the glass-transition temperature decreases and the α relaxation completely vanishes for the composite with 5 vol% OLC. As the α relaxation vanishes rapid enhancement of the mobility of the soft molecular segments in PU chains occurs and a decrease of the activation energy of electrical conductivity follows.Keywords: composites; dielectric properties; onion-like carbon