Under a temperature gradient, the direction of thermodiffusion of charged g Fe 2 O 3 nanoparticles (NPs) depends on the nature of the counter-ions present in the dispersion, resulting in either positive or negative Soret coefficient. Various counter-ions are probed in finely tuned and well characterized dispersions of citrate-coated NPs at comparable concentrations of free ionic species. The Soret coefficient S T is measured in stationary conditions together with the mass-diffusion coefficient D m using a Forced Rayleigh Scattering method. The strong interparticle repulsion, determined by SAXS, is also attested by the increase of D m with NP's volume fraction F. The F-dependence of S T is analyzed in terms of thermophoretic and thermoelectric contributions of the various ionic species. The obtained single-particle thermophoretic contribution of the NPs (the Eastman entropy of transferŜ NP) varies linearly with the entropy of transfer of the counter-ions. This is understood in terms of electrostatic contribution and of hydration of the ionic shell surrounding the NPs. Two aqueous dispersions, respectively with S T > 0 and with S T < 0 are then probed under an applied fieldH, an anisotropy of D m and of S T is induced while the in-field system remains monophasic. Whatever theH-direction (parallel or perpendicular to the gradients-T and-F), the Soret coefficient is modulated keeping the same sign as in zero applied field. In-field experimental determinations are well described using a mean field model of the interparticle magnetic interaction.