High-quality polyaniline with conductivity reaching 5 S cm⁻¹ has been prepared catalytically in acetonitrile using aniline, HCl, hydrogen peroxide, and 2 mol% of the copper catalyst methyl[tris(3-mesityl-1-pyrazolyl)borato]copper(II) chloride ([MeB(3-(Mes)Pz)₃]CuCl). The use of CuCl₂·2H₂O catalyst instead of [MeB(3-(Mes)Pz)₃]CuCl and the solvent effects were also investigated. Polyaniline samples were characterized using CHN analysis, UV-visible spectroscopy, FT-IR spectroscopy, bulk conductivity measurements, cyclic voltammetry and electrochemical impedance spectroscopy. Results of these mutually independent techniques are consistent and indicate the formation of good quality emeraldine form of polyaniline from the [MeB(3-(Mes)Pz)₃]CuCl catalyzed reaction in acetonitrile. The conductivity of the polymer prepared using [MeB(3-(Mes)Pz)₃]CuCl catalyst depends strongly on the medium of preparation which follows the order acetonitrile > 1 : 1 acetonitrile–water mixture > water. The polymer prepared using CuCl₂·2H₂O instead of [MeB(3-(Mes)Pz)₃]CuCl in a 1 : 1 acetonitrile–water mixture is of relatively poor quality and has a lower conductivity. HCl is also important for the formation of better quality polyaniline and higher yield.