We carried out thermodynamic study of the TlInSe2-TlGaTe2 system based on the data of physicochemical analysis. Based on thermodynamic analysis and concentration dependence of physical properties, it was found that there is anion-cation substitution in TlInSe2-TlGaTe2 system. Continuous series of (TlInSe2)1-x (TlGaTe2)x solid solutions is forming throughout entire concentration range. We determined dielectric characteristics of samples, their frequency dispersion and nature of dielectric losses. The results demonstrate that the dielectric dispersion in the studied crystals TlInSe2 and (TlInSe2)0.5(TlGaTe2)0.5 has a relaxation nature. Hyperbolic decline of loss tangent with increasing frequency from 50 kHz to 35 MHz indicates the loss of pass-through conduction in (TlInSe2)1-x(TlGaTe2)x solid solutions.