Spectroscopic ellipsometry was used to characterize the chain extension and optical properties of end-grafted deuterated poly(dimethylsiloxane) (d-PDMS) brushes on SiOx wafers exposed to liquid and supercritical carbon dioxide (CO2). The brush properties were manipulated by tuning the CO2 solvent quality over a large range from ideal gas conditions (non-solvent) to a near-Theta solvent by varying temperature and CO2 density. The chain extension determined by ellipsometry, for an average value of the pure component refractive index of CO2 in the film (average n(CO2) model), is in good agreement with that determined from neutron reflectivity. The d-PDMS chains extend into the CO, and the brush refractive index decreases as the solvent density is increased, especially at densities above the upper critical solution density for bulk PDMS in CO2. (C) 2004 Elsevier B.V. All rights reserved.