Phase separation of polyfluorene-based polymer blends is studied via photo- and electro-luminescence. By looking at Forster energy transfer, the authors prove that luminescence spectroscopy is a sensitive probe of the polymer environment, with a resolution of a few nanometers, which is advantageous over atomic force microscopy when the interpretation of the latter is complicated by a rich surface structure. De-mixing of the blends is found to occur upon exposure to a very poor solvent (acetone), and is attributed to enhanced segmental and chain dynamics of the macromolecular chains.