A new class of organic-inorganic hybrid thermoset copolymers has been prepared by ring-opening metathesis polymerization catalyzed with bis(tricyclohexylphosphine)benzylideneruthenium(II) dichloride (1). Dicyclopentadiene (DCPD) and norbornenylethyl polyhedral oligomeric silsesquioxane (1NB-POSS) and tris(norbornenylethyl)-POSS (3NB-POSS) with isobutyl pendent groups have been copolymerized at 60 degreesC over a range of POSS loadings. These copolymers contain small aggregates of 1NB-POSS, three to four molecules, at high loadings and uniform dispersions over all loadings of 3NB-POSS. The pendent group of 1NB-POSS decreases the cross-link density of the PDCPD matrix while 3NB-POSS increases the cross-link density. POSS incorporation has little effect on the glass transition temperature (T-g). Addition of 20 wt % 1NB-POSS decreases the T-g from 128 degreesC for PDCPD to 114 degreesC. Addition of 3NB-POSS has little effect on the T, over the range of POSS loadings. The stiffness, in tension and compression, is observed to decrease with increasing POSS loading. Although the yield stress for both systems decreases, the toughness also decreases. The decrease in toughness of the 1NB-POSS copolymers is attributed to a loss of irreversible damage with increased POSS loading. Although a similar decrease in toughness is observed for the 3NB-POSS copolymers, the change in toughness is attributed to a decrease in cohesive strength with 3NB-POSS loading.