Thermodiffusion, or the Soret phenomenon, is well understood in simple systems, but in multicomponent and polyvalent electrolyte systems the process becomes more complicated due to the coupling of fluxes. We experimentally investigate the time evolution of a concentration gradient generated by thermodiffusion of a polyelectrolyte (poly(sodium 4-styrene sulfonate), NaPSS) in a 1:1 supporting electrolyte. We also derive and solve the transport equations that are used to extract the Soret coefficient from the experimental observations. It is shown that NaPSS thermodiffusion in NaCl is strongly dependent on concentration, with almost 100% thermal separation in concentrations below 15 nmol L−1. Moreover, the results suggest that the supporting electrolyte can greatly influence the thermodiffusion of the polyelectrolyte, in some cases possibly even reversing the direction of the flux from the usual case of enrichment at the cold side.