This paper reports the effect of swift (80 MeV) oxygen (O+6) ion irradiation on the optical properties of CdTe thin films grown by conventional thermal evaporation on glass substrates. The films are found to be slightly Te-rich in composition and irradiation results no change in the elemental composition. The optical constants such as refractive index (n), absorption coefficient (alpha) and the optical band gap energy show significant variation in their values with increase in ion fluence. Upon irradiation the band gap energy decreased from a value of 1.53 eV to 1.46 eV whereas the refractive index (n) increased from 2.38 to 3.12 at lambda=850 nm. The photoluminescence spectrum shows high density of native defects whose density strongly depends on the ion fluence. Both analyses indicate considerable defect production after swift ion beam irradiation. (c) 2007 Elsevier B.V. All rights reserved.