Surfaces of multiwalled carbon nanotubes (CNT) were functionalized by treatment with strong acid mixture (purification) followed by modification with sodium dodecyl sulfate, poly(ethylene glycol) (PEG), and diglycidyl ether of Bisphenol A (DGEBA). Poly(ethylene terephthalate) (PET)-based conductive polymer composites were prepared by using these CNT by means of melt mixing with a twin screw extruder. Amount of carboxylic acid groups on the CNT surface increased after acid treatment but decreased with surface modification due to the consumption of these groups during the chemical reactions between the surface modifiers and CNT surface. Fourier transformed infrared spectroscopy (FTIR) and nuclear magnetic resonance analyses of the composites revealed the increase in the interactions between PET and CNT surface after treatment with PEG and DGEBA. Mechanical strength of the composites prepared with modified CNT were higher than that of the untreated CNT-filled composite owing to the enhanced interactions between PET and CNT. POLYM. ENG. SCI., 51:1286-1300, 2011. (C) 2011 Society of Plastics Engineers