The nanocasting method is a valuable tool for producing metal oxides with a well-defined nanostructure. However, the precise details on how the metal oxide isdeveloped inside the mesoporous silica template remain unclear. In this study, we clarify how nickel nitrate species are evolving to nickel oxide and how they are redistributed inside mesoporous SBA-15 particles as a function of heating temperature and surrounding gas atmosphere by a combination of in situ small-angle X-ray scattering, X-ray diffraction and thermogravimetric techniques as well as ex situ transmission electron microscopy and nitrogen physisorption measurements. The SBA-15 template was initially impregnated withNi(NO3)2·6H2O using the wet infiltration method. The results indicate an initial redistribution of the nickel nitrate salt located outside the pore system into the mesopores due to dissolution, while at temperatures of 110−150 °C (depending on which type of gas flow is used) the mobility of the salt is lost due to drying of the salt. Above 220 °C, the nickel nitrate decomposes, possibly via nickel hydroxynitrate, to NiO, forming nanoparticles inside the pore channels. The results shed light on the events occurring during the nanocasting process and can be used for further optimization of the fidelity of replication.