Organic acids were covalently bonded onto the pi-conjugated skeleton of surface-modified multi-wall carbon nanotubes (MWNTs) by treating them with carboxylic acid. Analysis of the Raman spectra of these MWNTs demonstrated that the acid treatment process increased the number of defects in the graphitic layers. Polyetherimide (PEI) was mixed with various amounts of well-dispersed MWNTs to form MWNTs/PEI composites. The dielectric properties of these composites were developed under different frequencies and the Lichtenecker logarithmic mixing rule was used to find the effective dielectric constant of the MWNTs. Because the MWNTs/PEI composites had high loss tangents, (Ba0.8Sr0.2)(Ti0.9Zr0.1)O-3 (BSTZ) ceramic powder was mixed with PEI and MWNTs to form BSTZ/MWNTs/PEI composites. The Lichtenecker logarithmic mixing rule was also used to predict the dielectric constants of these composites. The addition of BSTZ ceramic powder greatly improved the characteristics of the MWNTs/PEI composites, as the glass transition temperatures and dielectric constants increased. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.068204jes] All rights reserved.