Chemically deposited semiconductor polyaniline (PANI) thin films have been exposed to detect nitrogen dioxide gas. Because of the high electron receptor behavior, NO₂ molecules oxidize the conjugated polymer during the sensing process, leading to the modification of the electronic structure and consequently the change of the optical absorbance of PANI as a function of the toxic gas concentration. NO₂-induced optical transmittance changes of the polymer films can be varied between 5 and 80% for a NO₂ gas concentration of 3–50ppm. By using a parametric method, the heat of adsorption (Q) as well as the activation energy of adsorption (E_A) for NO₂ molecules on PANI surface are obtained from the corresponding adsorption kinetic curves. It is found that Q is of the order of 0.50–0.58eV, and E_A of 0.28–0.50eV, depending on the type of the PANI films and the NO₂ gas concentration. We show that the same kinetic expression can also be obtained from a reaction-diffusion coupled problem reported in literature by assuming a couple of the relations between the thermodynamic variables and kinetic reaction rates of the two models. The desorption process of NO₂ from PANI samples is kinetically irreversible in a dried air or N₂ during the experimental time, and slowly reversible if they have been kept in ambient air with a relative humidity greater than zero.