Mixed cation effect has been studied for thallium-silver thiogermanate glasses in the (Ag2S)(x)(Tl2S)(50-x) (GeS)(25)(GeS2)(25) system, where Ag2S fraction r = Ag2S / (Ag2S + Tl2S) = 0, 0.25, 0.5, 0.75, and 1.0. The glass transition temperature changes from 160 degrees C for r = 0 to 231 degrees C for r = 1.0 and exhibits a negative deviation from additivity, with a slight minimum T-g approximate to 157 degrees C observed for compositions in the range 0.1 <= r <= 03. The room temperature conductivity sigma(298) increases by 5 orders of magnitude with increasing silver concentration, ranging between 10(-8) S cm(-1) for the Tl-Ge-S system and 10(-3) S cm(-1) for the Ag-Ge-S glass with a minimum at sigma(298) approximate to 10(-9) S cm(-1). The activation energy decreases from 0.66 to 0.33 eV, respectively, with the maximum at E-sigma approximate to 0.69 eV. The minimum of both T-g and sigma(298) and the E-sigma maximum are observed in the same composition range, 0.1 <= r <= 0.3. The Tl-204 tracer diffusion coefficient at 170 degrees C decreases with silver concentration from 10(-10) cm(2) s(-1) for (Tl2S)(50)(GeS)(25)(GeS2)(25) glass to 10(-12) cm(2) s(-1) for (Ag2S)(50)(GeS)(25)(GeS2)(25) glass. The obtained results indicate a classical mixed cation effect. (C) 2014 Elsevier B.V. All rights reserved.