It is a well-known challenge to determine refractive index (n) from ultra-thin films where the thicknessis less than about 10 nm [1,2]. We discovered an interesting exception to this issue while characterizingspectroscopic ellipsometry (SE) data from isotropic, free-standing polymer films. Ellipsometry analysisshows that both thickness and refractive index can be independently determined for free-standing filmsas thin as 5 nm. Simulations further confirm an orthogonal separation between thickness and index effectson the experimental SE data. Effects of angle of incidence and wavelength on the data and sensitivity arediscussed. While others have demonstrated methods to determine refractive index from ultra-thin films[ 3,4], our analysis provides the first results to demonstrate high-sensitivity to the refractive index fromultrathin layers. (C) 2016 Elsevier B.V. All rights reserved.