Metal oxides are highly interesting materials for numerous industrially and economically important applications as a result of their semiconducting properties. Wide band gap metal oxides such as SnO2 and TiO2 are used, for example, in solar cells  or photocatalysis . Moreover, their ability to change conductivity when gaseous molecules are reacting with the surface makes them particularly applicable for chemoresistive portable gas sensors. A main drawback of metal oxide materials both in photocatalysis and as gas sensors, however, is the reaction of their surfaces with water vapor. In other words, changes in the relative humidity of the environment can significantly influence the performance of the metal oxide. This is one of the major shortcomings of SnO2-based gas sensors used, for example, in breath analysis. Experimentally, it has been shown that this can be overcome by doping of SnO2 with other metal atoms, such as Ti . In this project, density functional theory calculations have been utilized to simulate the formation of SnO2-TiO2 solid solutions demonstrating favorable distribution of Ti on the SnO2 surface, in particular on six-fold coordinated sites. Changes in the electronic structure of such SnO2-TiO2 surfaces leads to a destabilization of dissociatively adsorbed H2O species. A minimum in the H2O stability at low coverage has been found at a surface Ti-content of 25%. At high coverage, H2O is drastically destabilized when increasing the surface Ti-content from 0 to 30%. The overall minimum in the H2O stability can thus be assigned to a surface Ti-content of 25-30%. This gives a possible explanation for the minimum in cross-sensitivity to humidity found experimentally for Ti-doped particles .  J. F. Wager, Science 300 (2003) 1245.  A. Fujishima and K. Honda, Nature 238 (1997) 37.  A. Tricoli, M. Righettoni and S. E. Pratsinis, Nanotechnology 20 (2009) 31552.