A series of silica-zirconia mixed oxides were prepared by sol-gel routes where the zirconia content was varied from 9 to 75 mol%. The bulk and surface properties were analysed and compared with those of a sample where zirconia was deposited as an external layer on a silica surface thereby avoiding internal mixing of the component oxides. Spin-lattice relaxation measurements made by Si-29 MAS NMR provide a useful estimation of the relative size of domains containing silica only and thus the degree of mixing. High levels of silica distribution throughout the mixed oxide prevented crystallisation of zirconia phases even at high Zr: Si ratios. The extent of mixing at the surface and throughout the bulk is discussed in relation to the generation of both Lewis and Bronsted acid sites. Increased levels of mixing and heteroatom linkages throughout the bulk did not necessarily result in an increase in the density of surface Bronsted acid sites generated. (C) 2000 Academic Press.