Sn substitution for Ti in rutile TiO2 has been found to increase the photoactivity of the rutile by up to 15 times for the oxidation of acetone. The activity increases with increasing Sn in Ti1-xSnxO2 solid solution, and it peaks at a tin content of 0.075. Contrary to expectations, zeta potential measurements show that the poor photocatalytic activity of pure rutile is not due to the lack of hydroxyl groups on its surface. Photoexcited transient decay data relate the poor activity to the fast electron-hole recombination in rutile TiO2, and the addition of Sn to the rutile TiO2 lattice can effectively slow down the recombination rate. Diffuse reflectance UV-vis spectra of the solid solutions reveal a blue shift, indicating an increase in the bandgap of the semiconductor. Based on these experimental results, we believe that the origin of the high photoactivity of Ti1-xSnxO2 is the increase in its oxidation-reduction potential resulting from elevation of the solid solution's bandgap.