Segregation in crystals results in surface layer enrichment of selected lattice elements. The present work considers the phenomenon of segregation in solid solutions of TiO2 doped with donor-type elements, such as niobium and tantalum. The focus is on the effect of oxygen activity on segregation at elevated temperatures. It is shown that the effect of oxygen activity on the segregation-induced enrichment may be used for engineering of oxide materials in general and TiO2-based materials in particular with controlled surface composition that is required to achieve the desired reactivity and photoreactivity for energy conversion in general and solar energy conversion in particular. The development of the related surface engineering procedures requires correct determination of oxygen activity during processing and the segregation-induced concentration gradients within the surface layer.