The optical transmission and reflection at normal incidence of amorphous SbxSe1-x (0 less-than-or-equal-to x less-than-or-equal-to 0.9) thin films of different thicknesses (from 40 to 320 nm) were measured at room temperature in the wavelength region 500-1300 nm. From the obtained data, the absorption coefficient alpha, the extinction coefficient k, refractive index n, the optical energy gap E(g)opt and the valence band density of states g(i) were computed for different values of x. These constants were found to be independent of film thickness, but depend markedly on the value of the composition x. The analysis of the absorption coefficient data revealed the existence of two optical transition mechanisms, depending on the value of x, indirect transitions for SbxSe1-x thin films (x = 0. 1, 0.4, 0.5, 0.7, and 0.9) and a forbidden direct transition for x = 0.3. The optical energy gap E(g)opt was found to vary from 0.24 eV for (x = 0.9) to 1.92 eV for (x = 0). It was found that the high frequency optical constants n and k increase from 3.7 to 7.4 and from 0.32 to 0.64 respectively with increasing x from 0 to 0.9.