The electrochemical polymerization of aniline in a solution consisting of 0.2M aniline, 2M H2SO4 and 30% ionic liquid (v/v), 1-ethyl-3methylimidazolium ethyl.sulfate (EMIES), was performed at a constant potential of 0.70 V versus SCE. The spectra of IR and H-1 NMR of the resulting product, PAnIL, indicate that the ionic liquid (IL) was incorporated into polyaniline during the polymerization process. The cyclic voltammograms of the PAnIL demonstrate that the electrochemical behavior of the PAnIL in the solutions of pH < 3 is the same as that of polyaniline synthesized in the same conditions, but in the absence of the IL; however, the PAnIL in a solution of 0.3 M Na2SO4 with pH 12.0 still holds the redox activity. A significant result from the in situ spectroelectrochemical study shows that the absorption peak of the PAnIL film in a solution consisting of 0.3 M Na2SO4 and 20% IL (v/v) with pH 5.0 shifts from 509 to 890 nm, accompanied with the changes of colors from red-purple to violet, blue, green, greenish yellow and pale yellow, as the applied potential decreases from 0.70 to -0. 10 V versus Ag/AgCl with saturated KCI solution, and vice versa. The response time is within 2 s for the color change of the PAnIL film from red-purple to pale yellow. (C) 2007 Published by Elsevier Ltd.