Modern research techniques with radioactive isotopes were used to study the transport characteristics, (conductivity and diffusion) in chalcogenide membrane materials of the Ag₂S-As₂S₃-TlI system. It was demonstrated that, on raising the silver sulfide concentration in a glass, the total electrical conductivity grows by two orders of magnitude, which is due to the pronounced change of the band structure and to the increase in the electronic component of the conductivity. New chemical sensors for determination Tl ions in solutions were developed on the basis of these solid-state studies.