A novel process for preparing an ambient-temperature ionic liquid precursor (1-butyl-3-methylimidazolium chloride) was developed. The reaction of 1-methyl imidazole with butyl chloride was conducted in supercritical carbon dioxide (scCO(2)) and followed by in situ scCO(2) extraction for purifying the product, 1-butyl-3-methylimidazolium chloride. The yield of 1-butyl-3-methylimidazolium chloride could be > 90%, according to the reaction in a scCO(2) reaction environment. The residual reactants could be, in situ, removed completely using scCO(2) extraction after the required reaction time. The conversion of the reaction is dependent positively and strongly on the pressure. The kinetics of the reaction in scCO(2) could be described using the two-order irreversible reaction model. The reaction rate constant (k) is linearly dependent on the density of CO2 in the logarithmic coordinates. The H-1 nuclear magnetic resonance (H-1 NMR) spectral data of the prepared 1-butyl-3-methylimidazolium chloride product was satisfactorily consistent with its standard structure. The Fourier transform infrared (FT-IR) spectroscopy results show that the products are as pure as that made via the conventional reaction in the organic solvent/recrystallization method.