Amorphous bismuth‐tellurium oxide films have been prepared over a wide composition range by means of multicathodic reactive sputtering. Optical properties of the mixed‐oxide films are not algebraic combinations of the optical properties of the single‐oxide films. Relatively large quantities of bismuth oxide can be incorporated in virtually colorless mixed‐oxide films, because the bismuth oxide undergoes an unidentified transformation. In the high atomic ratio (Bi/Te) region, optical absorption is visible and attributed to the formation of the impurity‐stabilized &dgr;‐Bi2O3structure.