The synthesis of polyaniline (PANI) containing different carbon nanotubes (CNTs) by in situ polymerization is reported in this study. The samples were characterized by X-ray diffraction and scanning electron microscopy. Fourier transform infrared and ultraviolet-visible spectroscopy were used to determine the change in structure of the polymer/CNT composites. Thermogravimetric analysis showed that the composites had better thermal stability than the pure PANI. Photoluminescence spectra showed a blueshift in the PANI-single-walled nanotube (SWNT) composite. Low-temperature (77-300 K) electrical transport properties were measured in the absence and presence of a magnetic field up to 1 T. Direct-current conductivity exhibited a nonohmic, three-dimensional variable range hopping mechanism. The room-temperature magnetoconductivity of all of the investigated samples except the PANI-SWNT composite were negative; however, it was positive for the PANI-SWNT composite, and its magnitude decreased with increasing temperature. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 1016-1025, 2011