Variable angle spectroscopic ellipsometry has been used for determining the optical properties, in terms of the complex dielectric function in the near UV-VIS-near IR spectral range, and the thicknesses of thin fulgide films of type E-a-(2,5-dimethyl-3-furyl)-ethylidene(adamantylidene)succinic anhydride and (E)-2-[a-(2,5-dimethyl-3-thienyl)ethylidene]-3-isopropylidenesuccinic anhydride. The films had thicknesses in the range 28-40 nm and were spin coated onto silicon substrates. To simultaneously extract film thicknesses and optical properties of the films, several methods of analysis were employed in order to decrease correlation between the fitting parameters in the optical model of the structure. In agreement with previous absorbance measurements done by others on similar materials in liquid or solid form, it was found that the fulgides studied have several resonances in the wavelength region below 400 nm and that new resonances appear in the middle of the visible region upon UV exposure. The ellipsometric analysis, as well as atomic force microscopy studies, showed that the films were very smooth with a root mean square surface roughness <0.4 nm and that a small thickness change of the film takes place upon UV exposure. Furthermore, AFM studies revealed that large phase changes take place when the samples are stored in darkness for several days, indicating that the materials are not stable in thin film form. © 2001 Elsevier Science Ltd.