The strength development and healing of symmetric and asymmetric interfaces of amorphous cyclic olefin copolymers (COC) was studied by assessment of diffusion and lap-shear strength measurement of samples bonded at different temperatures, pressures and healing times. Amorphous COCs comprise copolymers of norbornene and ethylene whereby COCs with differing norbornene content have different glass transition temperature (T-g) and molecular weight. For similar thermal bonding conditions near Tg, the symmetric pairs have larger bond strength than asymmetric pairs. The activation energy calculated using Arrhenius model was independent of molecular weight and norbornene content. The free volume of COCs increased with norbornene content. In the asymmetric pairs, the adherend consisting of the COC grade with lower T-g and larger free volume dominated the diffusion compared to the other COC adherend. The penetration depth for molecular diffusion calculated using the Flory-Huggins theory was consistent with that estimated using the experimental values of the radius of gyration and bond strength. A new modified model that incorporates the combined effects of pressure, temperature and healing time could be used to predict the bond strength of both symmetric and asymmetric COC joints. Moreover, the new model facilitates determination of more reliable values of the activation energy. (C) 2012 Elsevier Ltd. All rights reserved.