The optical constants (absorption coefficient, refractive index, extention coefficient, real and imaginary part of dielectric constant) have been studied for a-Se80Te20-xPbx (where x = 0, 2,6, 10) thin films as a function of photon energy in the wave length range (500-1000 nm). It has been found that the optical band gap increases while the refractive index and the extinction coefficient (k) decreases on incorporation of lead in Se-Te system. The value of absorption coefficient (alpha) and the extinction coefficient (k) increases, while the value of refractive index (n) decreases with incident photon energy(.) The results are interpreted in terms of the change in concentration of localized states due to the shift in fermi level. (C) 2004 Elsevier B.V. All rights reserved.