TlInS2 single crystals are typical representatives of layered semiconductors. Layered crystals usually contain structural defects, such as vacancies and dislocations. The presence of these defects results in a high density of localized states near the Fermi level. Doping of these crystals allows one to tune their physical properties. The purpose of this work was to study the effect of partial transition metal (Cr) substitution for indium on the ac electric and dielectric properties of TlInS2 single crystals. Homogenous samples of TlInS2<Cr> single crystals at an chromium content of 0.5 and 1.0 mol.% were synthesized directly from the initial components. Single crystals of the TlInS2<Cr> compounds were grown by Bridgman method. It was shown that TlIn0.995Cr0.005S2 is crystallized in tetragonal structure. The crystal date for TlIn0.995Cr0.005S2 are following: a = 7.672; b = 7.723; c = 30.186 Å; b = 90°10¢; z =16. For TlIn0.99Cr0.01S2 a = 7.667; b = 7.715; c = 30.39 Å Investigated samples formed flat capacitors whose plane was perpendicular to the crystalline C-axis. Ohmic contacts of samples are made by Ag paste. Measurements of the dielectric coefficients of studied single crystals were performed at fixed frequencies in the range 5×104–3.5×107Hz by the resonant method using a TESLA BM 560 Qmeter. The electrical properties (loss tangent, real (e ) and imaginary (e ¢¢ ) parts of complex dielectric permittivity, and ac conductivity across the layers) of TlInS2 layered single crystals doped by Cr have been studied in the frequency range f = 5·104 to 3.5·107 Hz. The results demonstrate that the dielectric dispersion in the doped TlInS2 crystals has a relaxation nature. Partial substitution of Cr for indium in TlInS2 reduces the dielectric permittivity and changes the shape of the e ¢¢( f ) curve. Over the wide frequency range studied, the ac conductivity of the TlInS2 crystals doped by Cr varies as f 0.8, characteristic of hopping conduction through localized states near the Fermi level. The Fermi-level density of states (NF), the spread of their energies ( DE ), and the mean hop distance (R) and time (t ) have been estimated (Table). Table. Parameters of localized states in TlIn1-xCrxS2 single crystals obtained from high-frequency dielectric measurementsTlInS2 single crystals are typical representatives of layered semiconductors. Layered crystals usually contain structural defects, such as vacancies and dislocations. The presence of these defects results in a high density of localized states near the Fermi level. Doping of these crystals allows one to tune their physical properties. The purpose of this work was to study the effect of partial transition metal (Cr) substitution for indium on the ac electric and dielectric properties of TlInS2 single crystals. Homogenous samples of TlInS2<Cr> single crystals at an chromium content of 0.5 and 1.0 mol.% were synthesized directly from the initial components. Single crystals of the TlInS2<Cr> compounds were grown by Bridgman method. It was shown that TlIn0.995Cr0.005S2 is crystallized in tetragonal structure. The crystal date for TlIn0.995Cr0.005S2 are following: a = 7.672; b = 7.723; c = 30.186 Å; b = 90°10¢; z =16. For TlIn0.99Cr0.01S2 a = 7.667; b = 7.715; c = 30.39 Å Investigated samples formed flat capacitors whose plane was perpendicular to the crystalline C-axis. Ohmic contacts of samples are made by Ag paste. Measurements of the dielectric coefficients of studied single crystals were performed at fixed frequencies in the range 5×104–3.5×107Hz by the resonant method using a TESLA BM 560 Qmeter. The electrical properties (loss tangent, real (e ) and imaginary (e ¢¢ ) parts of complex dielectric permittivity, and ac conductivity across the layers) of TlInS2 layered single crystals doped by Cr have been studied in the frequency range f = 5·104 to 3.5·107 Hz. The results demonstrate that the dielectric dispersion in the doped TlInS2 crystals has a relaxation nature. Partial substitution of Cr for indium in TlInS2 reduces the dielectric permittivity and changes the shape of the e ¢¢( f ) curve. Over the wide frequency range studied, the ac conductivity of the TlInS2 crystals doped by Cr varies as f 0.8, characteristic of hopping conduction through localized states near the Fermi level. The Fermi-level density of states (NF), the spread of their energies ( DE ), and the mean hop distance (R) and time (t ) have been estimated (Table). Table. Parameters of localized states in TlIn1-xCrxS2 single crystals obtained from high-frequency dielectric measurements