Single crystals of Sb 2 Te 3 doped with Ag (c Ag ؍ (0-9) ؋ 10 19 cm ؊3 ) were prepared from the melt Sb 2؊x Ag x Te 3 (denoted by a) or the melt Sb 2 Ag x Te 3 (denoted by b). The reflectivity in the IR region, electrical conductivity, and Hall coefficient were determined for these crystals. From the reflection spectra values of the high-frequency dielectric constant, optical relaxation time, and plasma resonance frequency were obtained for crystals with various Ag contents. The dependencies of the real ( 1 ) and imaginary ( 2 ) parts of the dielectric function and the imaginary part of the energy loss function (Im(؊1/ )) on the wavenumber were also determined. From the 2 ( ) dependence at room temperature, it was concluded that in Sb 2 Te 3 (Ag) crystals the scattering mechanism of free carriers on the acoustical phonons prevails, but the participation of scattering on ionized impurities is also probable. The a-and b-type crystals of Sb 2 Te 3 (Ag) manifested practically the same values of transport coefficients and optical properties in the IR region. The determined values of the Hall coefficient showed that Ag atoms doping the Sb 2 Te 3 crystal structure behave as acceptors. The incorporation of 1 Ag atom into the crystals prepared from Sb 2؊x Ag x Te 3 and Sb 2 Ag x Te 3 melts increases the hole concentration by about 1.7 holes. This result is interpreted as the incorporation of Ag atoms into the crystal structure in the form of substitutional defects of Ag Sb with two negative charges. Part of the Ag atoms can form defects of the type [Ag Sb ؉ h • ], i.e., Ag Sb , or be incorporated in the fourlayer lamellae [Te-Ag 0.5 Sb 0.5 -Te-Sb 0.5 Ag 0.5 ] as uncharged defects. The Ag atoms incorporated into the Bi 2 Te 3 structure always behave as donors. The formation of Ag • i interstitials is proposed as well. The different behavior of Ag atoms in Sb 2 Te 3 and Bi 2 Te 3 crystals is probably associated with different parameters of the chemical bonds in these crystals, which is supported by the results of a semiempirical calculation.