In this work optical properties of Ta2O5 thin films with respect to heat treatment temperature were investigated. Ta2O5 thin films were prepared by sol-gel process using dip-coated method with a constant speed of 107 mm/min. Optical properties have been calculated from optical transmission measurements as a function of heat treatment temperature. The refractive indices and absorption coefficients were affected by heat treatment. The refractive index at lambda = 550 nm increased from 1.84 to 2.04 and absorption coefficient increased from 241 to 5668 cm(-1) when heat treatment temperature increased from 100 degrees C to 500 degrees C. The thickness of the film decreased from 272 to 190 nm and their optical band gap decreased from 3.68 +/- 0.09 eV to 3.51 +/- 0.08 eV for the film heated from 100 degrees C to 500 degrees C. (C) 1999 Elsevier Science B.V. All rights reserved.