Abstract A series of rare-earth metal disulfides LnS2 and diselenides LnSe2 (Ln = Sm, Gd, Tb, Dy, Ho, Er and Tm has been prepared by high-pressure high-temperature synthesis at 8(1) GPa. The dichalcogenides LnX 2 (X = S, Se) are found to crystallize in the α-LaS2 type with the monoclinic space group P121/a1 (no. 14). The crystal structure is a twofold superstructure of the ZrSSi aristotype and consists of puckered cationic [LnX]+ double slabs which are sandwiched by planar chalcogen sheets containing X 2 2– dumbbells.