Experiments on a nonstationary separation of nanometer-sizedMn0.5Zn0.5Fe2Oparticles of hydrocarbon-based ferrocolloids in a flat vertical thermal diffusion column are performed. By using a modified separation theory which accounts for a one-dimensional mixed (thermal and concentration) convection in the column, the Soret coefficient of lyophilized nanoparticles from the separation curves are calculated. It is shown that in a zero magnetic field particles are transferring toward decreasing temperatures. The thermal diffusion ratio&agr;Treaches a value&agr;T≈+20.A significant influence of a uniform magnetic fieldBon particle separation is observed. IfBis oriented along the temperature gradient&bnabla;T,a strong decrease in thermal diffusion coefficient takes place whereas the transversal fieldB⊥∇Tcauses an intensification of particle thermophoretic transfer. Both effects qualitatively well agree with theoretical predictions based on a hydrodynamic theory of particle thermomagnetophoretic motion. ©1998 American Institute of Physics.