Highly delaminated clay/poly(dicyclopentadiene) nanocomposites were prepared by in-situ, ring-opening metathesis polymerization of presonicated mixtures of the liquid dicyclopentadiene (DCPD) and organically modified Montmorillonite clays. Three nanocomposite series (using Montmorillonites: I-28,I-44pa) of increasing clay loadings (0.5, 1.0, and 2.0 wt%) and modified Montmorillonite PGW (0.5 wt%) were synthesized. The dispersed microstructure of the clays in the nanocomposites was characterized by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), small-angle X-ray scattering (SAXS), and high-resolution transmission electron microscopy (HR-TEM). All clays were highly delaminated and well dispersed within their host matrixes. The mean number of individual clay platelets per tactoid was predicted by fitting SANS data to the stacked-disk model and measured directly from HR-TEM images of a large number of tactoids in each sample. SANS results were in good agreement with HR-TEM for composites with low clay concentrations (0.5 wt%); however, deviations were observed at higher clay loadings (2.0 wt%). These deviations are discussed in terms of several factors not taken into account by the stacked-disk model: (a) long-range interactions, which become more important at high loadings; (b) departure of tactoid geometries from parallel stacks of rigid disks; and (c) the polydispersity of tactoid thicknesses. SAXS peaks were not present for most of the synthesized nanocomposites, suggesting a high degree of clay delamination was achieved. However, a broad peak was observed for the 2.0 wt% I-44pa clay nanocomposite, indicating the presence of some larger tactoids than in the other nanocomposites having a range of d-spacings. The HR-TEM measured distributions of clay platelet d-spacings in tactoids dispersed within the nanocomposites which were in the range from 8 to 20 Angstrom (12-15 Angstrom average), indicating that the quaternary ammonium ion pillaring agents were no longer present. These quaternary salts either decomposed during the high-temperature (260 degreesC) stage of curing or were not fully exchanged with the interlayer cations present between all platelet interlayers during the original clay modification. Clay/polyDCPD composites were also prepared using PGW clay modified by poly(ethylene glycol). This clay dispersed more rapidly, needing less sonication than the other clays. The resistance to delamination of all nanocomposites studied increased with greater clay loading under equivalent synthesis conditions.