Mixed lanthanum sulfide selenides LaS 2-xSe x (0<x<2) were obtained by metathesis reactions starting from anhydrous lanthanum chloride and alkali metal polychalcogenides. The LaS 2-xSe x compounds crystallize in space group P2 1/a, no. 14, and adopt the α-LnS 2 (Ln=Y, LaLu) structure type with a pronounced site preference for the chalcogen atoms. The mixed chalcogenides form a complete miscible series with lattice parameters a=820849 pm, b=413425 pm and c=822857 pm (β≈90°) following Vegard's rule. Raman signals indicate the presence of mixed X 2 2- dianions, a species rarely evidenced in literature, besides the well known anions S 2 2- and Se 2 2-. The band gaps of the LaS 2-xSe x compounds, determined by optical spectroscopy, decrease nearly linearly with increasing amount of selenium.