We report on the temperature-dependent adsorption of ethyl(hydroxyethyl)cellulose (EHEC) at the polystyrene-water interface revealed by ellipsometry. This technique allows separate determinations of the optical mean thickness, the average refractive index and the adsorbed amount of an adsorbed polymer film. The low optical contrast of the water-EHEC-polystyrene system makes measurements of these quantities difficult, but we show that they can be determined with sufficient accuracy by using a modern high precision ellipsometer.At room temperature, EHEC adsorbs sparsely on polystyrene surfaces (G≈1mg/m2), forming layers with an optical mean thickness that slightly exceeds 30 nm. Raising the temperature results in increasing adsorption and a simultaneous contraction of the adsorbed layer. The conjunction of these opposing trends leads to a strong decrease of the ellipsometric thickness near the bulk phase separation temperature at this surface. The adsorbed layer contains a fraction highly extended but dilute tails, reflected by its large hydrodynamic extension, as observed by photon correlation spectroscopy measurements (dh≈110 nm at 20°C). This fraction, however, does not significantly affect the interaction forces between two EHEC coated surfaces.