Ultrafine electrospun fibers of water-soluble polyaniline (PANI)/poly(ethylene oxide) (PEO) and multi-walled carbon nanotubes with attached carboxylic groups (c-MWCNTs) were successfully prepared by an electrospinning technique. Water-soluble PANI can be fabricated by a direct sulfonation of emeraldine salt form of PANI, which has been treated with chlorosulfonic acid in an inert solvent. The morphology and fiber diameter of water-soluble PANI/PEO/MWCNT fibers revealed that the composite fibers had a diameter ranging between 181 and 217 nm, with a general uniform thickness along the fibers. The X-ray diffraction data of these composite fibers were similar to those of PANI/PEO fibers, indicating the c-MWCNT is uniformly distributed in PANI/PEO matrix. The mechanical property of 5 wt % PANI/PEO/MWCNT composite fibers was about three times in magnitudes higher than that of PANI/PEO samples. The conductivity of 5 wt % PANI/PEO/MWCNT composite fibers at room temperature is three orders in magnitude higher than that of PANI/PEO specimens. These results demonstrate that the addition of a small number of c-MWCNTs to a PANI/PEO matrix can form a conducting network in well dispersed composite fibers, thus increasing their electrical conductivity. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012