We report on the optical ozone sensitivity of a polyaniline (PANI) film prepared on the inner wall of a polystyrene cuvette by chemical oxidation of aniline from aqueous solution. The PANI film showed changes of optical absorbance in the presence of 50–100 ppm ozone in air at room temperature. The absorbance change was largest at wavelengths in the range of 500–800 nm. The absorbance changes brought about by ozone was partially reversible after removal of ozone from the atmosphere. These results suggest that PANI is a potential material for optical ozone detection.