A high efficiency of thermoelectric conversion can be achieved by using materials with a maximum figure of merit Z, i.e. materials with a high Seebeck coefficient S, high electrical conductivity a and low thermal conductivity K. Investigations of transport phenomena in alloys of the (TIBiS₂) _1-x(2PbS)_x system have shown that in solid solutions of type (A³B⁵C₂⁶) _1-x (2A⁴B⁶)_x cation substitution according to the scheme 2A⁴⁽⁺²⁾ → A³⁽⁺¹⁾ + B⁵⁽⁺³⁾ leads to a strong decrease in lattice thermal conductivity K_L. In an area (X = 0.50) of the K_L = f(C) curve the lattice part of the thermal conductivity of (TlBiS₂)_1-x(2PbS)_x alloys decreases to 0.26 W m^-1 K^-1 which is approaching the theoretical minimum. As a result, the dependence of the thermoelectric figure of merit on composition Z = f (C) of alloys of the (TlBiS₂) _1-x(2PbS)_x system passes through a maximum in the area X = 0.50, exceeding by ∼25% the respective value of Z for lead sulfide at room temperature.