Based on the results of X-ray phase analysis, an isothermal section of the quasi-ternary system Ag2S-CdS-SnS2 at 298 K was constructed. The existence of the Ag2CdSnS4 compound was confirmed, and a new quaternary phase Ag2CdSn3S8 was discovered. The crystal structure of both compounds was refined using X-ray powder method. Ag2CdSnS4 crystallizes in the orthorhombic space group Cmc2(1) with the unit cell parameters a=0.41015(3), b=0.70224(4), c=0.66946(4) nm (R(I)=0.0550). The Ag2CdSn3S8 crystal structure refines well in both the tetragonal structure of the rhodostannine type (space group I4(1)/a, a=0.76163(6), c=1.0771(2) nm, R(I)=0.0750) and the cubic structure of the chalcospinel type (space group Fd3m; a = 1.07635(2) nm, R(I) =0.0781). The phase diagram of the Ag8SnS6-CdS section was constructed, which is of the eutectic type. The eutectic point coordinates are ~47 mol.% CdS and 1082 K.