Highly transparent and conductive polyaniline-cellulose acetate composite films were cast from m-cresol solutions of protonated polyaniline and cellulose acetate. The following polyaniline protonating agents were used : camphor sulfonic acid, phenylphosphonic acid, dibutyl phosphate, and dioctyl phosphate. The films were plasticized with a mixture of phthalic and phosphoric acid esters added to the solution used for the casting. The addition of plasticizers not only improved the flexibility of the composite films but also significantly lowered the percolation threshold, which is 0.05 wt % (expressed as emeraldine base) for phenylphosphonic acid protonated polyaniline in cellulose acetate matrix.