An unmixing-based data fusion technique is used to generate images that have the spatial resolution of Landsat Thematic Mapper (TM) and the spectral resolution provided by the Medium Resolution Imaging Spectrometer (MERIS) sensor. The method requires the optimization of the following two parameters: the number of classes used to classify the TM image and the size of the MERIS ldquowindowrdquo (neighborhood) used to solve the unmixing equations. The ERGAS index is used to assess the quality of the fused images at the TM and MERIS spatial resolutions and to assist with the identification of the best combination of the two parameters that need to be optimized. Results indicate that it is possible to successfully downscale MERIS full resolution data to a Landsat-like spatial resolution while preserving the MERIS spectral resolution.