Spectroscopic ellipsometry from 0.73 to 4.75 eV was used to study the optical properties of epitaxial GaAs0.9-xNxSb0.1 layers with x=0.00, 0.65, 1.06, 1.45 and 1.90%. The ellipsometric experimental spectra were fitted using a multilayer model employing the model dielectric function to describe the GaAs0.9-xNxSb0.1 optical response. We have identified the Gamma-point E-0, E+, and E-# transitions of GaAs0.9-xNxSb0.1 and have determined the effect of nitrogen on the respective transition energies. We have demonstrated that a lower N content can provide an equal E+-E-0 energy splitting for GaAs0.9-xNxSb0.1 with respect to GaAs1-xNx. (C) 2010 Elsevier By. All rights reserved.