This article exposes a study of the rheological properties of silica−zirconia sols in acidic conditions, which are used for the development of nanostructured optical fibers by an original “inverse dip-coating” method. This new generation of optical fibers, which contains zirconia nanocrystals dispersed inside an amorphous pure silica core, must present a homogeneous core to allow good waveguiding properties. Thus, good control of the rheological properties of the silica−zirconia sols at its origin is needed to optimize the development process. After having described the chemical synthesis of the sols, the principle of the rheological measurement is then explained and a short reminder of a few and essential rheological notions is provided. The evolution of the rheological behavior of the sols is then studied as a function of the time and thus as a function of the polymerization advancement. Finally, in order to extend and stabilize the development process of these fibers, an estimation of the sol viscosity is done by taking into account the different viscosity designation notions that are inherent to the flow regime of the fluid.