Ferrite nanoparticles from Fe(NO3)(3) and Me(NO3)(2) (Me = Ni, Cu, Zn) aqueous solutions could be synthesized continuously with a flow reactor at 673 K, 30 MPa, and 4 s residence time. The particles were characterized by TEM, XRD, and ICP to obtain sizes, crystal structures, lattice parameters, and Me/Fe molar ratios. Solid-solution nanoparticles of MeFe2O4 and gamma-Fe2O3 with a cubic spinel structure and an average particle size under 10 nm were obtained. Conversion of Fe 31 was more than 0.97 at the given residence time, and conversions of Me 21 increased from 0 to 0.62 with increasing residence time. The particle size increased with increasing residence time, and no significant difference in divalent cations was observed. The Me/Fe molar ratio in the obtained solid-solution nanoparticles of MeFe2O4 and gamma-FeO3 increased with increasing residence time and KOH molality. The Me/Fe molar ratio also increased in the order Zn < Cu < Ni at a given residence time, and this trend could be explained on the basis of ZnO, CuO, and NiO solubilities. The increase of the size and the Me/Fe molar ratio in the obtained particles with increasing residence time shows that, in the formation mechanism, primary solid-solution particles with low Me/Fe molar ratios probably nucleated and, after surface dissolution of the particles, dissolved Fe3+ recrystallized on the surface with incorporation of Me2+.