Abstract: The most optimal membrane samples were synthesized, tested, and selected and a comprehensive study of the transport and analytical parameters of membrane materials for potentiometric sensors based on chalcogenide glasses in the TlI–Ag₂Se–As₂Se₃ system was conducted. Thallium-selective sensor samples with membranes of the composition: 20 mol % TlI–30 mol % Ag₂Se–50 mol % As₂Se₃ and 25 mol % TlI–25 mol % Ag₂Se–50 mol % As₂Se₃, demonstrated the best parameters of the electrode function (57–58 mV/pTl), a detection limit of 3 × 10^–6 M, and the best characteristics of selectivity with respect to a number of heavy metal cations.