The water vapor sorption properties of PMA/P4VP Complex were examined. The amount of water vapor absorbed by PMA/P4VP Complex was higher than that by the pure components, especially in the low and middle relative humidity regions. This means that the polarization due to polyion complex formation has a positive effect on its sorption property. With the good dye susceptibility and hygroscopicity of PMA/P4VP Complex, we attempted to modify the surface of PET film by in-situ reaction of PMA and P4VP grafted PET film in methanol. The PMA/P4VP Complexed PET showed higher dyeability and hygroscopicity, but worse anti-electtostatic property than P4VP grafted PET.