Polypropylene/carbon nanotube/exfoliated graphite nanoplatelet (PP/CNT/xGnP) composites have been fabricated to evaluate their electromagnetic interference shielding effectiveness (EMI SE) and soundproofing. An EMI SE of 36.5 dB at 1250 MHz was measured for the 80/10/10 wt % PP/CNT/xGnP composite; its sound transmission loss was more than 5 dB higher than that for pure PP at low frequencies (520-640 Hz). These results indicate simultaneous EMI SE and soundproofing. Transmission electron microscopy was used to study the microstructure and to probe synergetic effects between the CNTs and xGnPs. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3947-3951, 2013