The structure and properties of poly(acrylic acid) (PAA)-doped polyaniline (PAn) (or termed PAA/PAn blend) were investigated by X-ray diffraction, dynamic mechanical analysis, conductivity measurement, ultraviolet-visible-near-infrared spectroscopy, infrared spectroscopy, and atomic force microscopy. The PAA-doped PAn subchains can not align in an ordered fashion as can those of HCl-doped PAn. Therefore, the PAA/PAn blends are amorphous. The blend exhibits a glass transition temperature higher than that of a PAn film plasticized with the same amount of 1-methyl-2-pyrrolidone (NMP), because the doped PAn subchains are more rigid and contain less NMP nearby. During the glass transition, some of the protonated N atoms of PAn are deprotonated due to the increased thermal motion of PAA-doped PAn subchains or, thermodynamically, decreased miscibility between PAA and PAn on heating; the protons generated can combine with the counterions, COO-, to become COOH. Thus partial undoping of PAn occurs and the conductivity of the blend (2.9 x 10(-4) S/cm for the composition of the mole ratio 1/1) drops at temperatures higher than T-g. When the blend is cooled to room temperature, some of the undoped PAn subchains are redoped. For the PAA-doped PAn film, the confinement of carboxylic acid groups on the flexible polymer chains of PAA leads to a nonuniform and inefficient doping, such that some of the acid groups are unable to participate in the the doping and PAA is mainly distributed in the region where PAA-doped PAn subchains aggregate; and the undoped PAn forms another phase, though the acid/base interaction promotes the miscibility between PAA and PAn.