Disclosed herein is a method of evaluating the performance of an ion-exchange
film. In the method, small-angle scattering curves for the ion-exchange film are
obtained by an X-ray measuring apparatus that can detect X-rays scattered at small
angles with respect to the axis of an X-ray applied to film. From the positions
of the peaks on the small-angle scattering curves and the X-ray intensities at
these peaks, the molecular structure of the ion-exchange film is determined, thereby
to evaluate the performance of the ion-exchange film.