The paper presents the results of experimental studies of the electro-optical properties of β-Tl1-xCuxInS2 (0 X 0,015) single crystals. It was found that in measurements of the induced birefringence, the transmission of the polarizer - crystal - analyzer (P-C-A) system is nonzero both for the presence of an external electric E = 0 and in its absence, which is apparently associated with the presence of internal fields arising in crystals during their growth ().