γ-Fe₂O₃@SiO₂, TiO₂@SiO₂, and α-Fe₂O₃@SiO₂ core–shell nanoparticles were synthesized by a surfactant-free hydrothermal method in a continuous flow dual-stage reactor. Uniform silica shells of 2 nm thickness were obtained when grown on γ-Fe₂O₃ and TiO₂ nanoparticles while thicker and more irregular shells were deposited on α-Fe₂O₃ due to low pH.