© 2017 Author(s). We investigate the change of the valence band energy of GaAs 1-x Bi x (0<0.025) as a function of dilute bismuth (Bi) concentration, x, using x-ray photoelectron spectroscopy (XPS). The change in the valence band energy per addition of 1 % Bi is determined for strained and unstrained thin films using a linear approximation applicable to the dilute regime. Spectroscopic ellipsometry (SE) was used as a complementary technique to determine the change in GaAsBi bandgap resulting from Bi addition. Analysis of SE and XPS data together supports the conclusion that ∼75% of the reduction in the bandgap is in the valence band for a compressively strained, dilute GaAsBi thin film at room temperature.