Branched polymeric niobia-silica (NS) mixed-oxide sols with a Nb:Si molar ratio between 0.33 and 0.8 were made by acid-catalyzed sol−gel synthesis and characterized using small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). The growth rate of NS sols after addition of a niobium alkoxide precursor to a prehydrolyzed silica sol in alcohol under highly acidic conditions was monitored. The results indicated a fractal structure, with radii of gyration and fractal dimensions that increased simultaneously as a function of time. The radii of gyration were between 2 and 5.5 nm, whereas the fractal dimensions were in the range of 1.5−2.0 within experimental error. The experimental data are indicative of growth via a diffusion-limited cluster aggregation (DLCA) mechanism on a time scale of hours. The dominant growth mechanism in the very early stage of reaction must have been different. Sols with higher Nb:Si molar ratios grew faster, which is due to the higher reactivity of Nb compared to Si alkoxides. Dilution and control over temperature can be employed to control the size of the sols precisely.