Thin films of Bi10SbxSe90-x (x greater than or equal to 35, 40, 45) of different composition and thickness, were deposited on glass substrates by vacuum evaporation. Optical absorption measurements show that the fundamental absorption edge is a function of glass composition, film thickness and annealing temperature. The optical absorption is due to indirect electronic transitions. The value of the optical band gap was found to increase with thickness and decreasing the antimony content and with increasing temperature of heat treatment. The validity of the Urbach rule was investigated and the respective parameters estimated. X-ray diffraction was used to obtain an insight into the structural information.