The mutual miscibility of five binary ethylene/norbomene copolymers of the NxE1-x/NyE1-y type where x and y represent the norbornene content in the first and second copolymers, respectively (with x/y of 0.36/0.52, 0.36/0.56, 0.36/0.62, 0.46/0.60 and 0.46/0.66), in five different compositions was studied by rheology and differential scanning calorimetry. Most of the blends do not obey the principle of time-temperature superposition, i.e., they can be considered as thermorheologically complex, and hence immiscible. The experimental data from theology were compared to a recent lattice cluster theory. The theoretical miscibility diagram correctly predicts the experimental cases for most blend compositions except for the very asymmetric compositions. (c) 2007 Elsevier Ltd. All rights reserved.