It has recently been shown that the viscosity of polymer/water mixtures can be accurately predicted with an ideal linear mixing rule based on molecular van der Waals surfaces. Because of the Stokes-Einstein relation, a similar mixing rule may also be applicable to ionic diffusion coefficients at infinite dilution in polymer/water mixtures. This assumption is confirmed for the example system KDP/polyol/water at various temperatures. Moreover, the impact on electrolyte concentration is calculated and shows that the surface-based mixing rule can also be used as a good approximation for the ionic diffusion coefficient for finite electrolyte concentrations.