The microstructure of electron beam deposited Bi3Se100 thin films (where x varies from 0 to 16 at.%) was investigated. The morphology of crystallization for in situ thermally annealed and electron beam heated Bi16Se84 films was investigated using transmission electron microscopy. Selected area electron diffraction was used to characterize different phases observed during the crystallization process where the crystalline Bi2Se3 phase was separated. Optical absorption measurements were carried out on as-deposited BixSe100-x films of different compositions. It was found that the mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap decreases with increasing Bi content. The effect of thermal annealing on the optical properties of Bi16Se84 films was investigated. The study indicated that Bi16Se84 films have low threshold energy for amorphization, high optical absorption coefficient and optimum contrast between the amorphous and the crystalline states. The decrease in the optical gap is discussed on the basis of amorphous-crystalline transformations