The aim of the paper is to evaluate the Soret coefficient S-T of colloidal particles in ferrofluids from measurements of an unsteady particle separation in a flat thermodiffusion column. The column theory is modified taking into account MHD effects of thermal convection affected by a concentration buoyancy force. It is shown that the Hartmann effect, not only in hydrocarbon based colloids but also in ionic magnetic fluids of relatively high electric conductivity, does not influence significantly the particle separation dynamics. From measurements, positive values of S-T of surfacted particles in tetradecane based colloids are calculated. An anisotropy of the Soret effect in the presence of a uniform magnetic field is experimentally established. The obtained results agree qualitatively well with the authors hydrodynamic theory of particle thermomagnetophoresis. (C) 2000 Elsevier Science Ltd. All rights reserved.