Solid solutions of Ti1-xZrxO2 (0.00 < x less than or equal to 0.10) exhibit higher photocatalytic activity than pure anatase TiO2 for the degradation of acetone in air. The activity increases with increase of zirconium substitution in the TiO2 lattice until nominal x = 0.075. The results of polycrystalline X-ray diffraction analysis, BET surface area, XPS, and zeta potential measurements show that the increase in activity is related to changes in the lattice parameters and cell volume. A mechanism is proposed suggesting that lattice O-2(-) and O- species ionosorbed on surface are responsible for the increased activity. The synthesis of the solid solutions and the effects of zirconium addition on phase constitution, lattice parameters, BET surface area, and surface hydroxyl groups are also discussed.