ABSTRACT Detailed model study of THz generation by CO 2 laser down-conversion in pure and solid solution crystals GaSe 1-x S x is carried out for the first time. Both forward and backward co llinear interactions of common (eo-e, oe-e, oe-o, oo-e, ee-o) and original (ee-e, oo-o) types are consid ered. Possibility of realization, phase ma tching angles and figure of merits are estimated for line mixing within 9 P m and 10 P m emission bands, as well between them. Dispersion properties of o- and e-wave refractive indices and absorption coefficients for GaSe, GaS and GaSe 1-x S x crystals were preliminary measured by THz-TDS, approximated in the equation form and then used in the study. Estimated results are presented in the form of 3-D figures that are suitable for rapid analyses of DFG parameters. The most efficient type of interaction is eo-o type. Optimally doped (x = 0.09-0.13) GaSe 1-x S x crystals are from 4 to 5 times more efficient at limit pump intensity than not doped GaSe crystals. Keywords: GaSe, GaSe