Polyaniline/carbon (Pani/C) composites were synthesized by oxidative polymerization of aniline in the presence of carbon material at different pH values. Composites obtained possess identical composition (56 wt% of polymer as determined by CHNS elemental analysis) but differ in morphology and porous structure. Biomass-derived activated carbon having specific surface area of 1740 m2 g−1 was used as carbon component. Porous structure was characterized by low-temperature nitrogen adsorption at 77 K. The electrochemical behavior of the samples was investigated by cyclic voltammetry in a three-electrode cell in different electrolytes: aqueous 1 M H2SO4 and 1 M solution of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) in acetonitrile (ILE). Based on cyclic voltammetry data mechanism of polyaniline transformations in ILE involving BMIM+ as a proton source was proposed. Symmetric supercapacitors (SC) were assembled with Pani/C electrodes and ILE. Study of SC by the set of electrochemical techniques and investigation of the electrode structure prior and after stability tests performed by Fourier transform infrared spectroscopy allowed to reveal the main regularities of SC performance and evolution of material properties.