Bismuth oxide (Bi ₂O ₃) thin films with thickness in the range 20-160 nm have been deposited by d.c. reactive magnetron sputtering of Bi in an atmosphere Ar:O ₂ (1:1), onto single crystalline p-GaSe (Cu) substrates. The optical constants, n and k, of oxide films have been determined from the analysis of the polarization ellipse of the reflected radiation from outer surface of Bi ₂O ₃/p-GaSe structures. In the wavelength range 400-800 nm the refractive index of nanometric Bi ₂O ₃ films onto GaSe(Cu) decreases from 2.10 to 1.78 and it seen to increase at decreasing sample thickness. In order to determine the interaction mechanism between semiconducting oxide film and GaSe surface, the spectral characteristics of photocurrent through Bi ₂O ₃/p-GaSe junction and optical absorption in the range 400-800 nm have been examined. As resulted from respective analyses, Bi ₂O ₃ film generates new valence bonds, which contribute to the increase in the density of localized states at Bi ₂O ₃/p-GaSe (Cu) junction interface.