The mutual miscibility diagram is determined for binary blends of two ethylene/norbornene random copolymers, and their monomer sequence distribution is investigated by C-13 NMR spectroscopy. The miscibility changes abruptly at a norbornene content of 50% where NMR measurements indicate a change in microstructure. When both copolymers have a norbornene content higher than 50%, all blends are miscible within the composition ranges available. A simplified version of the lattice cluster theory (LCT) is applied to this system. The computations indicate that the chain stiffness (arising from the rigidity and steric interactions of the norbornene units) exerts significant influence on the miscibility of these blends. Experimental miscibility diagrams agree reasonably with the LCT predictions.