Tris(8-hydroxyquinoline) aluminum (Alq 3 ), used as light-emitting layer in electroluminescent (EL) devices, has been synthesized. The thin films have been deposited by thermal evaporation on quartz and silicon substrates. The optical constants of Alq 3 films have been determined using spectroscopic ellipsometry (SE) over a wide spectral range. The influence of light exposure on the optical properties of Alq 3 films has been studied using ellipsometry, photoluminescence and infrared spectroscopies. We find the structural modification in Alq 3 films as indicated by the refractive index and absorption coefficients after exposure to light in air ambient.