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Titanium dioxide (TiO2) has received much attention as a photocatalyst, specifically in applications that require a mechanically robust thin film. TiO2 is particularly useful because it does not absorb visible light, making it well suited for coatings on glass. Photocatalytic activity of TiO2 films is strongly dependent on the substrate and it has been well established that sodium diffusion from glass has a negative effect on this activity. While the prevention of sodium poisoning is possible through the use of a precoating, this requires an additional coating and/or calcination step. Other remedies, such as acid treatment of the glass surface, are also time consuming. Therefore, it is a more attractive option to negate the effects of sodium diffusion without a separate processing step. In this paper, we examined the effects of silver, cobalt, copper, gallium, molybdenum, and tantalum doping on the prevention of sodium poisoning of sol–gel TiO2 films by comparing the photocatalytic activities on glass and SiO2 precoated glass. While sodium poisoning degraded the photocatalytic activity of undoped TiO2 films by 70%, it was only 10% for Mo- and Ta-doped TiO2 films. Molybdenum was superior to other dopants in terms of photocatalytic activity, both in the presence of sodium and in a sodium-free environment.