The principle of the mole-ratio method utilizing magnetophoretic velocimetry was demonstrated for the first time. The feasibility of the magnetophoretic mole-ratio method was studied for the determination of the stoichiometry of the complexes formed between a hydrophobic phosphate ligand adsorbed in mesoporous hydrophobic silica particles and paramagnetic metal ions of Co(II), Tb(III), and Dy(III) in aqueous solutions. The present method is a nonspectroscopic method and carried out by a simple magnetic device.