Results on thermodiffusion of poly(ethylene oxide) and colloidal boehmite (gamma-AlOOH) rods in ethanol/water mixtures are presented. Data were obtained using thermal diffusion forced Rayleigh scattering. The sign of the Soret coefficient of the boehmite rods changes from positive to negative with increasing water content, i.e., at sufficiently high water content the colloidal particles move to higher temperatures. The sign of the Soret coefficient of the poly(ethylene oxide) in ethanol/water mixtures is negative, i.e., the poly(ethylene oxide) molecules move to higher temperatures, whereas in pure water the sign is positive. To our knowledge this is the first time that a sign change has been observed for polymers in solution. The analysis of the static light scattering on poly(ethylene oxide) allows the determination of the preferentially solvating solvent. In the investigated concentration range the preferentially solvating solvent is ethanol, in spite of being the poorer solvent for poly(ethylene oxide). (C) 2003 American Institute of Physics.