This letter deals with an adaptation of the ac-electrogravimetry technique to extract separately the dynamic properties of H+ and water in Nafion nanometric thin films (average thickness of 400 nm). An original theoretical approach was developed to extract the representative parameters from ac-electrogravimetry data. The concentration change of the exchanged species and the diffusion coefficient of the protons in a Nafion nanometric thin film (D = 0.5 脳 10鈥? cm2 s鈥? at 0.3 V vs SCE) were estimated for the first time according to the applied potential. The conductivity value of Nafion thin films was calculated from the Nernst鈥揈instein equation using diffusion coefficients and concentration values extracted from ac-electrogravimetry data. The calculated conductivity results agree well with the experimental proton conductivity values of Nafion thin films.