Exfoliated poly(dicyclopentadiene) (pDCPD)-montmorillonite (MMT) nanocomposites were synthesized via intergallery-surface-initiated ring opening metathesis polymerization (ROMP). This is the first example of in situ polymerization of pDCPD from clay intergallery surfaces using ROMP. Grubbs catalyst was immobilized on the surface of MMT clay modified with vinylbenzyl dimethyloctadecyl ammonium chloride (VOAC), and DCPD polymerized from the clay surface while simultaneously crosslinking to form a thermoset nanocomposite in a one-pot reaction. X-ray diffraction and transmission electron microscopy analysis indicated that the resultant nanocomposites exhibited exfoliated morphologies with heterogeneous clay platelet distribution. Conventional bulk-initiated nanocomposites containing VOAC modified MMT were also synthesized as a comparison, and these resulted in nanocomposites with intercalated morphologies. The differences between the morphologies demonstrated that growing polymer chains from the initiator sites on the intergallery surface of the clay platelets pushed the platelets apart during the polymerization of the intergallery-surface-initiated nanocomposites, aiding in the exfoliation process. Compression testing indicated that the intergallery-surface-initiated nanocomposites led to improvements of up to 50% in the compressive Young's Modulus, while the bulk-initiated nanocomposites at the same clay loadings did not exhibit improved properties. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 89-97, 2012