Thermal evaporation technique was used to prepare As20Se80−x Tlx films from bulk materials;( $5\leqslant x\leqslant 35$  at.%). The effect of TI addition on the thermoelectric properties and mobility activation energy of As-Se-Tl chalcogenide semiconductors has been studied in the homogeneous glass-forming region through temperature range (300–380 K). The thermoelectric power, TEP had a positive sign over the whole temperature range investigated, indicating p-type conductivity for As20Se80 −x Tlx films. TEP activation energy, $\Delta E_{s}$ could be calculated from TEP measurements. It was found that $\Delta E_{s}$ decreases with increasing Tl-content and is found to vary between 0.814 and 0.517 eV. The mobility activation energy, $\Delta E_{Q}$ could be calculated.