Spin-dependent transport properties of magnetic nanostructures have been investigated by means of magneto-thermogalvanic voltage measurements: the ac voltage response to an ac temperature oscillation is measured for various magnetic nanostructures under dc current. The samples studied include Co/Cu multilayered nanowires, homogeneous Ni nanowires and cobalt clusters embedded in copper films. The magnetic field dependence of this signal is always larger than the magneto-resistance (MR) and may be anisotropic even when the MR is not. A thermodynamic argument introduces spin mixing as the main process measured by this novel thermoelectric measurement technique. This effect is not observed in magnetite as can be justified by the absence of an accessible second spin channel.