The optical properties of amorphous GeSe₂, GeSe₄, GeSe₂Tl and GeSe₄Tl films, prepared by thermal deposition from their bulk glasses, have been studied. The respective effect of adding thallium to a-GeSe₂ and a-GeSe₄ results in increasing the density, d, from 4.16 to 6.2 g cm^-3 and from 4.34 to 5.72 g cm^-3, the refractive index, n, from 2.42 to 2.78 and from 2.43 to 2.62, whereas decreasing the glass transition temperature, T_g from 395 to 192 degrees C and from 165 to 134 degrees C, and the optical energy gap, E_g, from 2.07 to 1.54 eV and from 2.02 to 1.48 eV. The obtained results are discussed in the light of the proposed structural models of amorphous Ge-Se. The decrease in the energy gap with the introduction of thallium involves a change in the microstructure accompanied by a decrease in the chemical ordering of the stoichiometric compound a-GeSe₂ and an increase in the amount of chemical disorder in the composition of a-GeSe₄. The pronounced decrease of the gap (about 25%) is attributed to an alloying effect rather than to a hypothetical increase of the gap state density.