The influence of added sucrose on the phase composition of iron oxide nanoparticles was studied in the Fe2O3/SiO2 nanocomposites. The nanocomposites were prepared by impregnation of mesoporous silica SBA-15 with iron nitrate containing various amounts of sucrose and their subsequent annealing at 1100 and 1200 °C. The phase composition of the iron oxide nanoparticles in the prepared nanocomposites was analyzed by X-ray diffraction and 57Fe Mössbauer spectroscopy on polycrystalline samples. Profile analysis of the X-ray diffraction maxima was used to determine the apparent crystallite size of the different polymorphs. The content of β-Fe2O3 in the samples increased with an increasing amount of sucrose, and no β-Fe2O3 was observed in the samples without sucrose. These samples contained only ε-Fe2O3 and hematite. The apparent crystallite size was found to be similar for both hematite and β-Fe2O3. Therefore, sucrose was identified as the primary variable, which directs the ε-Fe2O3 transformation to either hematite or β-Fe2O3. Our findings shift the way we understand the β-Fe2O3 formation from ε-Fe2O3 in the Fe2O3/SiO2 system from the size-directed to the starting material-directed transformation.