Single wavelength ellipsometry measurements at Brewster's angle and in multiple incident media provide a powerful technique for characterizing ultrathin (<20-30 nm) polymer films. Only one ellipsometric parameter (i.e. amplitude ratio) is obtained at Brewster's angle since phase shift is 90 degrees. By conducting the experiments in different ambient media, simultaneous determinations of a film's thickness and refractive index for ultrathin polymer films are possible at Brewster's angle. Poly(tert-butyl acrylate) (PtBA) Langmuir-Blodgett films serve as a model system for the simultaneous determination of thickness and refractive index (1.45 +/- 0.01 at 632 nm). Thickness measurements on films of variable thickness agree with X-ray reflectivity results +/- 0.8 nm. The method is also applicable to spincoated films where refractive indices of PtBA, polystyrene and poly(methyl methacrylate) are found to agree with literature values within experimental error. Published by Elsevier B.V.