The interaction between water and resist film is a very important subject to be studied in order to establish the feasibility of 193-nm immersion lithography. The water diffusion into 193nm resist films was measured by using Quartz crystal microbalance method and it showed the slow saturation after the quick water uptake in the early stage of dipping in water. Diffusion coefficient was approximated by polynomial function of diffusion time. The water diffusion was well elucidated by the single variable of diffusion coefficient, which reflects the conditions of bake or pre-soak process and molecular properties such as molecular weight. The analysis is shown to be useful to evaluate the diffusion mechanism and to develop materials for immersion lithography.