The bismuth oxide films evaporated from bulk Bi 2O 3 are shown to vary in stoichiometry. The as-evaporated low rate (1–5 Å/sec) films are microcrystalline and bismuth rich, relative to Bi 2O 3, and their optical absorption edge broadens and shifts to lower energies. High rate (15–25 Å/sec) films are morphous and oxygen-rich with an absorption edge shifted to higher energies. Thermal decomposition of the Bi 2O 3 during evaporation produces the variations in film stoichiometry. The high temperature δ-Bi 2O 3 observed in the as-evaporated low rate films and thermally treated amorphous films indicates the melt and the films are structurally similar. Thermal treatment of the low rate films results in the formation of the β-form. Comparison of X-ray and stoichiometry results suggests that β-Bi 2O 3 be expressed as β-Bi 2O ±3 x , where x is the deviation from trioxide stoichiometry.