Nanostructured polyaniline (PANI) counter electrodes (CEs) with different doping levels have been fabricated using in-situ electrochemical polymerization under different doping H sub(2)SO sub(4) concentration for applications in dye-sensitized solar cells (DSSCs). The effects of the H sub(2)SO sub(4) concentration on properties of the obtained PANI CEs, such as morphology, degree of polymerization, doping level of the SO super(2) sub(4) super(-) counter anions, oxidization state, and electrochemical activity for I super(-) sub(3) reduction, are thoroughly investigated using different techniques, such as scanning electron microscope (SEM), ultraviolet-visible light spectroscopy (UV-vis). X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). When using D149 dye-sensitized nanocrystalline TiO sub(2) photoanode, DSSC based on PANI CE polymerized with 0.35 M H sub(2)SO sub(4) shows the best photovoltaic performance with a solar-to-energy conversion efficiency up to 5.57%, which is competitive to that of Pt CE-based solar cells (6.00%). This is because PANI CE polymerized under such conditions exhibits high catalytic activity for I super(-)/I super(-) sub(3) redox reaction because of the large surface area, high degree of polymerization/oriented structure, high doping level, and high emeraldine base oxidization state. Thus, our results can help to prepare the PANI CEs with a better DSSC performance and, more important, to better understand the operation mechanism of PANI CEs.