The copolymerization behavior and the apparent cure kinetics of epoxy resin E51 containing the expanding monomer 3,9-di(5-norbornene-2,2)-1,5,7,11-tetraoxaspiro [5,5] undecane (NSOC, norbornene spiro orthocarbonate), were investigated by using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. Evidence for random copolymerization of the E51-NSOC system was obtained by dynamic mechanical analysis and electron microscopy. The ring-opening reaction of the epoxy resin was found to be first order with an activation energy of 62.0 kJ/mol and the reaction with the expanding monomer NSOC was second order with an activation energy of 121.3 kJ/mol. The comonomers were joined with an ether linkage. Longer copolymerization sequence lengths were obtained with higher NSOC contents in the copolymer.