The ozone-sensitive visible/near-IR absorbance changes of poly(2-chloroaniline) (P(2ClANI)), poly(N-methylaniline) (P(NMeANI)) and polyaniline (PANI) films were investigated. These three polymer films, which were assumed to be present partially in a protonated form and partially in a reduced form, were prepared by chemical oxidation of the monomers followed by treatment with the reducing agent disodium disulfite. The P(2ClANI) film was sensitive to ozone at concentrations of down to 2.5 ppm, but saturation of sensitivity started to appear above 50 ppm. By contrast, the PANI and P(NMeANI) films were sensitive to ozone in a concentration range of 50–100 ppm without showing saturation of sensitivity.