Optical absorption edge spectra have been measured by diffuse reflectance spectroscopy on a complete suite of rare earth sesquisulfide compounds of varying structure type. Optical band gaps were extracted from these data. The observed band gaps tend to vary with crystal structure type but many compounds with the same structure have similar band gaps independently of the particular rare earth ion. The ordered alpha-structure has a substantially lower band gap than the defect gamma-structure for all compounds examined except for La2S3. (c) 2005 Elsevier Ltd. All rights reserved.