Using a three-current model (i.e. heat, spin-up and spin-down electrons) the thermodynamics of irreversible processes predicts that a temperature gradient gives rise to a spin current on large scales, in particular under the conditions used to measure the spin Seebeck effect. Diffusive currents on large scales are common in thermochemistry. The proportionality between the diffusion current and the gradient of the chemical potential is known as the Soret effect or thermophoresis. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim