The stabilization of cobalt cations in zirconium–silica fiberglass materials was studied by X-ray photoelectron spectroscopy, ultraviolet visible diffusion reflectance spectroscopy, and a differential dissolution phase analysis. It was found that the commercial Zr–Si fiberglass material contained a layer of hydrated silicon (the depth of 6 nm) on the surface of the glass fibers. Modification of the fiberglass material with cobalt led to its distribution in the fibers mainly in the Co²⁺ state. It was shown that 90% of cobalt was stabilized on the surface and in the hydrated silicon layer as Co²⁺ cations in tetrahedral oxygen coordination, while the remaining 10% was distributed non-uniformly in the bulk of the fibers as Co²⁺ cations in octahedral oxygen coordination.