For the development of mid-infrared fiber-optical elements,one needs light-stable,flexible materials that are transparent within this spectral band.Solid solutions of silver and monadic thallium halides prove to be the most suitable crystalline media for these needs.We study the light stability variation of high-purity AgCl_(1-x)Br_x(0 ≤ x ≤ 1) and Ag_(1-x)Tl_xBr_(1-x)I_x(0 ≤ x ≤ 0.05) crystals by measuring the optical transmission change as functions of the composition and UV exposure time.The former is executed in a broad spectral range from6500-350cm~(-1),within which we choose three wavelengths to trace the transmission change.For thalliummonoiodide-containing samples,an effect is observed that we assumed to be translucence. For the development of mid-infrared fiber-optical elements, one needs light-stable, flexible materials that are transparent within this spectral band. Solid solutions of silver and monadic thallium halides prove to be the most suitable crystalline media for these needs. We study The light stability variation of high-purity AgCl_ (1-x) Br_x (0≤x≤1) and Ag_ (1-x) Tl_xBr_ (1-x) I_x (0≤x≤0.05) crystals by measuring the optical transmission change as functions of the composition and UV exposure time. The former is executed in a broad spectral range from 6500-350cm -1, within which we choose three wavelengths to trace the transmission change. For thalliummonoiodide-containing samples, an effect is observed that we assumed to be translucence.