The morphology and physicochemical properties of poly(4-vinylpyridine)-filled microfiltration membranes have been examined. These membranes, which were prepared by photoinitiated grafting of up to 125 mass% of 4-vinylpyridine into the pores of polypropylene (PP) microfiltration membranes, were characterized in terms of the amount of poly(4-vinylpyridine) incorporated (graft yield), ion-exchange capacity, water content, and thickness. The morphology of samples of the grafted membranes dehydrated by freeze substitution was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. While the ion-exchange capacities of the grafted membranes are a function of graft yield, ranging to 4.0 meq/g of dry membrane, the water contents of the grafted membranes in the free base form are essentially independent of graft yield. The porosity of the grafted membranes was shown to be almost the same as that of the starting base polypropylene membranes (60-80%). However, protonation of the grafted poly(4-vinylpyridine) was shown to lead to a further and very substantial increase of the void volume of the membranes as measured by their water content. The thickness of the grafted membranes was found to increase linearly with increasing incorporation of polyvinylpyridine. Ionization of the polyelectrolyte was shown to cause a further systematic increase in thickness which was partly reversible with reversion to the unprotonated form. These changes in thickness are attributed to the stretching of the mesh of the substrate microfiltration membranes.