The double potential step method was used to study the electric properties of polyaniline (PAn) film in acidic aqueous solutions. The resistance and capacitance of bulk film changed with prolonging of the reduction time and/or shifting of reduction potential in the negative direction. And also a shoulder appeared in the cathodic chronoamperometric curve indicate the two-stage reduction process of PAn. These experimental results have been satisfactorily explained by using an improved heterogeneous model, which takes the merits of both the percolation theory and the electrochemistry stimulated conformational relaxation model. This new model overcomes some difficulties that the previous two models may meet, and especially can quantitative explain the change of electric properties of PAn film in the reduction process. (c) 2005 Elsevier Ltd. All rights reserved.