The creation of nanomaterials requires simultaneous control of not only crystalline structure and composition but also crystal shape and size, or morphology, which can pose a significant synthetic challenge. Approaches to address this challenge include creating nanocrystals whose morphologies echo their underlying crystal structures, such as the growth of platelets of two-dimensional layered crystal structures, or conversely attempting to decouple the morphology from structure by converting a structure or composition after first creating crystals with a desired morphology. A particularly elegant example of this latter approach involves the topotactic conversion of a nanoparticle from one structure and composition to another, since the orientation relationship between the initial and final product allows the crystallinity and orientation to be maintained throughout the process. Here we report a mechanism for creating hollow nanostructures, illustrated via the decomposition of β-FeOOH nanorods to nanocapsules of α-FeO, γ-FeO, FeO, and FeO, depending on the reaction conditions, while retaining single-crystallinity and the outer nanorod morphology. Using in situ TEM, we demonstrate that the nanostructured morphology of the starting material allows kinetic trapping of metastable phases with a topotactic relationship to the final thermodynamically stable phase.