New membrane materials were studied for thallium (I)-selective chemical sensors based on chalcogenide glasses in the TlI-Ag₂S-As₂S₃ system. Using these studies on the radioactive tracers diffusion (^108mAg Ee Cyrillic sign ²⁰⁴Tl) and electrical conductivity, an ionic conductivity of σ_ion = 10^-7 Ohm^-1 cm^-1 (20°C) was found for the glass composition 27%TlI-20%Ag₂S-53%As₂S₃. This composition of chalcogenide glass was selected as a membrane material for the Tl-selective chemical sensor displaying a value of the electrode function of 57 mV/pTl and a detection limit of 3 × 10^-6 mol l^-1.