Ternary lanthanum sulfide selenides LaS 1.9-xSe x (0 < x < 1.9) were obtained by reaction of the elements in a potassium bromide flux. The ternary compounds adopt the CeSe 1.9 structure type (tetragonal, space group P4 2/n, No. 86) and form a complete solid solution series without indications of ordered superstructures. The lattice parameters of the samples follow Vegard's rule and are found in the range of 915 ≤ a ≤ 943 pm and 1644 ≤ c ≤ 1703 pm. A pronounced ordering of the distribution of chalcogen atoms is observed which can be attributed to charge density and size effects. The existence of three different types of dichalcogenide anions, namely S22-, Se22- and mixed (SSe) 2-, is evidenced by Raman spectroscopy.