ABSTRACTOxygen uptake by the rare earth chalcogenides takes place through a series of ordered compounds. At high oxygen concentrations the distinct and nearly stoichiometric oxychalcogenides, Ln202X (X = S, Se, Te) appear. For the chalcogenides of the larger rare earths there appears an ordered oxygen-containing beta structure, Ln10X14OxX1-x (X = S, Se). The vibrational spectrum of the trigonal oxysulfide structure contains four infrared and four Raman bands (2 A2u + 2 Eu + 2 Alg + 2 Eg). Band wavenumbers across the La to Lu series vary linearly with unit cell volume. The Raman bands are sharp indicating a high degree of order in the intermediate compounds. The Raman bands of the beta structure are remarkably sharp indicating that this compound also has a highly ordered structure. Known data plus synthesis data are combined to form not-impossible phase diagrams for the larger rare earth sulfide systems. The effect of oxygen in both series of compounds is to produce small wavenumber shifts rather than high wavenumber oxygen impurity bands.