The optical properties of Ge Se(y-x) Tl-x thin-film samples of different thicknesses are studied as a function of the thallium contents. Transmittance and reflectance measurements in the range of (400-2600 nm) of various thicknesses range of(114-618.3 nm) were used to calculate the optical band gap E-g(opt) the width of localised states and optical constants n and k. It is shown that the optical band-gap decreases and the width of the tail increases with the increase in the Tl content. The allowed optical transitions were found to be indirect transitions. From the transmission spectra, the relationship between the refractive index n and the wavelength (lambda) at different n contents is derived. The increase in the refractive index with the increase in the n content is attributed to the increase in the valency density of the current carriers. Analysis of the refractive indices has yielded optical dielectric constant (epsilon (x)).