A new role for metal Sn in TiO₂ crystallization from the amorphous precursor is observed. Amorphous TiO₂ precursor films synthesized to crystallize into the anatase polymorph upon annealing in air produced rutile polymorph when metal Sn is incorporated in sufficient concentration. When Sn is incorporated as an oxide, only anatase TiO₂ forms. Based on this observation, a mechanism is proposed where metal Sn acts as an oxygen scavenger and produces an oxygen-deficient environment in the amorphous TiO₂ matrix which favors the rutile structure upon annealing.