The strength of fusion bonds between polyamide 12 plaques has been investigated as a function of processing conditions, using the double cantilever beam geometry to measure the mode I critical strain energy release rate, GC. Bonding was carried out using an instrumented press, which allowed systematic variation of the bonding time, the holding pressure, the interface temperature and the initial difference in temperature between the plaques. GC was generally found to increase with hold time and interface temperature. However, for a given hold time, hold pressure and estimated interface temperature, the highest GC were obtained under nonisothermal conditions, that is, when the initial temperatures of the plaques were different. Based on fractographic analysis and observations of the effect of the hold pressure, it is concluded that enhanced wetting is an important factor in the improved efficiency of non-isothermal bonding.