A series of cycloolefin copolymers (COCs) comprised of ethylene and norbornene units were characterized by NMR, WAXD, and DSC. The investigated compositional range was from 35.6 to 79.0 mol % of norbornene. Materials of norbornene content of less than 50 mol % were found to contain blocks of ethylene units and also sequences of alternating ethylene/norbornene units. In these cases the stereoregularity of the chain was relatively high. Materials of norbornene content of more than 50 mol % were also found to contain alternating sequences but additionally showed components of a more random nature including blocks of norbornene units of varying lengths. The stereoregularity of the chain was relatively low in these cases. Differences in the WAXD of the materials were related to these differences in chemical architecture. All the materials showed one T-g which varied linearly with the composition of the copolymer.