An optical study of vacuum evaporated (10 (4) Pa)Ge20Se70-xIn10Bix (x = 2, 4, 6, 8, 10) thin films is reported in the present work. The optical constants viz. refractive index (n) and extinction coefficient (k) have been accurately determined by envelope method using transmission spectra in the range of 400-1800 nm. The dispersion of the refractive index of a-Ge-Se-In-Bi thin films is analyzed in terms of the Wemple-DiDomenico single-effective-oscillator model. Refractive index increases with increase in Bi content, whereas optical band gap (calculated using Tauc plot) decreases significantly from 1.63 eV to 0.87 eV. The variation in the optical behavior for different thin films is explained on the basis of defect states and the decrease in average bond energy of the system. The dielectric constants (epsilon(r) and epsilon(i)) and optical conductivity (sigma) of the thin films are also reported. (C) 2008 Elsevier B. V. All rights reserved.