Abstract Blends of poly(vinylidene fluoride) (PVDF) with a perfluorosulfonate ionomer, Nafion®, have been prepared and examined in terms of the crystallization kinetics of the PVDF component. In blends of PVDF with Na+-form Nafion®, the rates of bulk crystallization, as observed by DSC, and the spherulitic growth rates of the PVDF component, as observed using optical microscopy, were found to be very similar to that of pure PVDF. This behavior was attributed to the course phase separation of Na+-form Nafion® from PVDF and melt incompatibility of the physically cross-linked ionomer with the crystallizable component. In this segregated state, the PVDF component of the blend is allowed to crystallize in pure phases that are isolated under the influence of Nafion®. In contrast, when the ionomer was exchanged with more weakly interacting quaternary alkylammonium counterions, a decrease in both the rate of bulk crystallization and spherulitic growth was observed. Furthermore, the crystallization kinetics of PVDF in these blends was found to be dependent on the counterion size; as the size of counterions associated with the Nafion® component increased, the rate of crystallization decreased. This behavior was attributed to a weakening of the electrostatic interactions in the ionomer phase and thus an increase in the extent of phase mixing with the larger ions.