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, Tg from 395 to 192 degrees C and from 165 to 134 degrees C, and the optical energy gap, Eg, 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.