Hollow nanoscale polymer capsules have found wide applications in drug delivery and catalysis. The use of nanocapsules and related materials for drug delivery, as nanoreactors, or as platforms for chemical separations requires control over cargo entry and exit and installation of diverse chemical functionality in the capsule interior. Despite the diversity of methods for nanocapsule preparation, methods for efficiently functionalizing the core of hollow nanocapsules are scarce. We report here a versatile and modular approach for the synthesis of hollow polymeric capsules that can be readily and diversely functionalized in the core. Capsules functionalized with pyrene, ferrocene, amines, and carboxylic acids have been prepared. The capsules are characterized using dynamic light scattering, fluorescence spectroscopy, and transmission electron microscopy. These core functionalized capsules can selectively extract hydrophilic cationic or anionic dyes into organic solutions based on the charge complementarity of the core functionality. The synthetic approach described here is modular and permits facile control of the interior as well as exterior properties of the nanocapsules.