The fractal properties of xerogels of hydrated zirconium dioxide ZrO ₂, which are synthesized by precipitation from a zirconyl nitrate solution at various pH values, are studied. Small-angle neutron scattering data and low-temperature nitrogen adsorption data indicate the strong effect of the acidity of the medium on the fractal dimension of xerogels and the size of constituent monomer particles. It is shown that this effect is also characteristic of xerogels of hydrated hafnium dioxide HfO₂, which is a chemical analogue of hydrated zirconium dioxide.