The phase diagram of the system Ag4SSe–As2Se3 is studied by means of X-ray diffraction, differential thermal analyses and measurements of the microhardness and the density of the materials. The unit-cell parameters of the intermediate phases 3Ag4SSeAs2Se3 (phase A) and Ag4SSe2As2Se3 (phase B) are determined as follows for phase A: a=4.495 , b=3.990 , c=4.042 , α=89.05, β=108.98, γ=92.93; for phase B: a=4.463 , b=4.136 , c=3.752 , α=118.60, β=104.46, γ=83.14. The phase 3Ag4SSeAs2Se3 and Ag4SSe2As2Se3 have a polymorphic transition α↔β consequently at 105 and 120C. The phase A melts incongruently at 390C and phase B congruently at the same temperature.