The structural and electronic properties of selected compositions of SnxTi1-xO2 solid solutions (x = 0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard's law in the SnxTi1-xO2 system. Our results also account for the fact that chemical decomposition in SnxTi1-xO2 system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of SnxTi1-xO2 solid solutions. Sn substitution for Ti in TiO2 increases the oxidation-reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron-hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in SnxTi1-xO2 solid solutions. (C) 2003 Elsevier B.V All rights reserved.