GaSe1-xSx (x = 0, 0.01, 0.05, 0.13, 0.22, 0.29, 0.44) crystals were grown by modified vertical Bridgman method that provided (0001) crystal plane orientation perpendicular to growth axis. Absorption coefficient is minimal (decreased by up to 3 times) in the THz range at the optimal S-doping of 3 weight % (x = 0.13). Increased S content causes the absorption coefficient for e-wave to become frequency independent in the main part of the spectrum from 03 to 3 THz. A narrow line-width rigid phonon absorption peak E ''((2)) at 1.79 THz arises and then declines in intensity with the doping level, in parallel with a shift towards shorter wavelength. (C) 2015 Elsevier B.V. All rights reserved.