Thermal properties of an epoxy system containing diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcyclohexane (1,3-BAC) as curing agent modified with poly(ether imide) (PEI) were studied using both dynamic mechanical (DMA) and thermogravimetric (TG) analysis. The effect of thermal degradation in air and in vacuum on the dynamic mechanical and thermogravimetric properties for the DGEBA/1,3-BAC/PEI system was investigated. The results showed that this system can be considered miscible for PEI contents up to 10 phr (phr: number of parts of PEI per hundred parts of the DGEBA epoxy resin), and also that even very small contents of PEI in the blends reduced the thermal stability of the material. Thermal degradation during a short period of time of this material both, in air or in vacuum yields a decrease of thermal stability and an increase of the glass transition temperature with respect to the material without degradation.