The thermal degradation for a system containing a diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcyclohexane (1,3-BAC) has been studied using dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The DMA results showed that the peak value of the dynamic loss factor (tan delta), the glass transition temperature (T-g), and the dynamic storage modulus (E’) above T-g changed considerably with increasing thermal degradation, while E’ below T-g only underwent a moderate change with increased thermal degradation. There was no appreciable change in the beta transition temperature (T-beta) with thermal aging time. DSC results showed the same trend for T-g as with DMA. A shift of T-g to higher temperature with increasing aging times was found, but a decrease in T-g for longer aging times was also observed. This could be attributed to thermooxidative degradation in the oven. From these results it can be argued that thermal degradation during the stage prior to the onset of the severe degradation involves structural changes in the epoxy system, such as further crosslinking and loss of dangling chains in the crosslinked network.