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.