An experimental program to determine the mode I critical energy release rate (GIc) of the T650/AFR-PE-4/FM680-1 material system is reported. Two forms of GIc are determined over the range of 20 to 350 ◦C, the area method critical energy release rate GIc and the inverse method critical energy release rate (GIc). The value of GIc was found to increase with increasing temperature. The inverse method is determined to be a very effective method of determining GIc over the entire range of temperatures. Inverse modeling was completed using the finite element method, coupled with a novel Discrete Cohesive Zone Element, to determine GIc over the range of crack advance in a given specimen. The element is described in detail, as well as the metrics used by the inverse algorithm. The FE models, subsequent to inverse modeling, accurately reproduce the experimental load-displacement curves. They therefore provide a capable analysis method, as well as a material system constitutive relation that contains a range of appropriate properties for use in the design and analysis of T650/AFR-PE-4/FM680-1 joints.