The effect of substituting thallium oxide with silver oxide on some physicochemical properties of glasses in the xAg₂O-(0.2 - x)Tl ₂O · 0.8B₂O₃ system at x = 0, 0.04, 0.08, 0.12, 0.16, and 0.20 has been investigated. It has been established that, upon this substitution the concentration dependences of the glasses conductivity activation energy and electroconductivity are of an extreme-type character, whereas similar dependences of other properties (microhardness, glass formation temperature, and density) are nearly linear. The study of the nature of current carriers and transport numbers in accordance with the Hittorf technique has shown that silver ions and protons (resulting from dissociation of water always present in glasses) participate in the transport of electric current. As was concluded based on the experimental data, thallium ions do not participate in electricity transport. The rule of additivity of electroconductivity is not valid for glasses in the Ag₂O-Tl₂O-B₂O ₃ system.