The structure of titania/zirconia colloids has been investigated using small-angle neutron scattering (SANS). The colloids were produced by: (i) hydrolysis of a mixture of titanium and zirconium alkoxides and peptization of the resulting hydrolyzate with nitric acid (homogeneous) and (ii) hydrolysis of a titanium alkoxide and peptization of the resulting hydrolyzate with aqueous zirconium(IV) nitrate solution (heterogeneous). The final titania/zirconia and metal oxide/nitrate mole ratios were 16.0 and 10.0, respectively. The sols were concentrated by evaporation, dried under controlled conditions and redispersed in D 2O/H 2O mixtures. FT-Raman spectra of the sols, and XRD powder patterns from the gels, showed that the material was crystalline anatase and amorphous zirconia. Both TEM and XRD line broadening indicate that the crystallite size of the dried titania gels is ∼8 nm. The results of SANS contrast variation experiments are described. The minimum-contrast points for the homogeneous and heterogeneous colloids, determined by two different methods, gave similar results which differed significantly from the expected value, due to the sorption of nitrate counterions and hydroxyl species on the surface of the colloids. In both systems, the scattering at minimum contrast was consistent with a network of unidimensional zirconia, with a typical diameter of ∼1.5 nm. At full contrast, the homogeneous colloids have fractal dimensions (d f of 1.6) similar to those from static light scattering, that is. consistent with an open, extended, chamlike aggregate structure. However, the heterogeneous colloids have higher fractal dimensions (d f of 2.2-2.4), due to zirconia crystallites packing the interstices between the titania crystallites, that is. consistent with a loosely packed, spherical structure. The apparent fractal dimension of the heterogeneous colloids in H 2O (d f of 2.4) is higher than that observed in D 2O (d f of 2.2). whereas no such effect is observed for the homogeneous colloids. These results infer that, in the homogeneous colloids, the zirconia is segregated within the matrix of the titania crystallites (on the ∼1 nm scale), whereas, in the heterogeneous colloids, the zirconia is segregated on the surface of the titania crystallites (on the ∼10 nm scale).