The mode I fracture toughness of a single co-cured and two secondary bonded joint systems were determined using the double cantilever beam test. The initiation values of fracture toughness from the PTFE film insert and a mode I crack-tip were considered as well as propagation values. It was found that the starting defect had a large influence on the initiation values for fracture toughness. It was also found that the two secondary bonded systems predominantly resulted in cohesive failure while the co-cured joiailed interfacially. Thermogravimetric analysis coupled with mass-spectrometry was used to show how moisture in the composite prepreg and adhesive affected the toughness of the joints. Microscopy methods were used to gain further insight into the damage mechanisms of the three joint systems.