The combination of multinuclear solid-state NMR and powder X-ray diffraction has been applied to characterize the octahedron-shaped crystalline nanoparticle products resulting from an inverse micelle synthesis. Rietveld refinements of the powder X-ray diffraction data from the nanoparticles reveal their general formula to be (H 3 O)Y 3 F 10 • xH 2 O. 1 H magic-angle spinning (MAS) NMR experiments provide information on sample purity, as well as serving as an excellent probe of the zeolithic incorporation of atmospheric water. 19 F MAS NMR experiments on a series of monodisperse nanoparticle samples of various sizes yield spectra featuring three unique 19 F resonances, arising from three different fluorine sites within the (H 3 O)Y 3 F 10 • xH 2 O crystal structure. Partial removal of zeolithic water from the internal cavities and tunnels of the nanoparticles leads to changes in the integrated peak intensities in the 19 F MAS NMR spectra; the origin of this behaviour is discussed in terms of 19 F longitudinal relaxation. 19 F-89 Y variable-amplitude cross-polarization (VACP) NMR experiments on both stationary samples and samples under conditions of MAS indicate that two distinct yttrium environments are present, and based on the relative peak intensities, the populations of one of the two sites is closely linked to nanoparticle size. Both 19 F MAS and 19 F-89 Y VACP/MAS experiments indicate small amounts of an impurity present in certain nanoparticles; these are postulated to be spherical amorphous YF 3 nanoparticles. We discuss the importance of probing molecular-level structure in addition to microscopic structure, and how the combination of these characterization methods is crucial for understanding nanoparticle design, synthesis, and application.