Neutron diffraction with isotopic titanium substitution allows the nature of the cation sites to be determined beyond nearest-neighbor distances for the first time in a sol-gel derived titania-silica glass. Reverse Monte Carlo (RMC) modelling of the results shows unequivocally that titanium substitutes for silicon within the homogeneous glass network, existing mostly as TiO4, with a minority TiO6 component. The fundamental scientific interest, and wide-ranging technologically significant properties of titania additions to silica, demand an understanding of the precise nature of the titanium site. (C) 2004 Elsevier B.V. All rights reserved.