Optical properties in terms of the complex-valued dielectric function were determined for spin-coated films of a Rose Bengal derivative using variable angle of incidence spectroscopic ellipsometry in the visible and infrared wavelength regions. In addition, the thickness and roughness of the films were determined and related to the solution concentration of Rose Bengal. Switching between two different oxidation states of the Rose Bengal derivative was investigated. The two states were chemically induced by exposure to vapors of hydrochloric acid and ammonia, respectively. A substantial and reversible change of the optical properties of the films was observed. (C) 2011 Elsevier B.V. All rights reserved.