Composite materials are used more and more for aeronautical applications and they have to be as thermally stable as possible. The thermostability of carbon-fiber/epoxy-cyanate composites elaborated with an autoadhesive and autoextinguish prepreg were tested, Dynamical and isothermal aging tests were carried out to evaluate the composite thermal stability. Thermal degradation products were identified by chromatography/mass spectrometry analysis and the results obtained were compared with data known on the material network structure. The physicochemical network structure evolutions and the thermal aging data are correlated with the interlaminar shear strength (ILSS) mechanical results. For epoxy-cyanate composites, cracking appears after a longer time of aging than for epoxy composites. This new epoxy-cyanate material isothermal stability seems to be good and particularly good if it was postcured after processing. The comparison of chemical, mechanical, and crack formation results obtained by accelerated aging tests allowed us to determine models to predict long-term behavior.