The thermal stability of high-performance thermoplastic matrix composites is studied by thermogravimetric analysis. The study is focused on the degradation behavior, under different environments, of continuous carbon-fiber-reinforced composites with poly(ether-ether ketone) (PEEK) matrix. The experimental results are analyzed by two different methods. In one method a phenomenological kinetic model is developed and compared with the degradation behavior measured in isothermal and constant heating conditions. In the second method, a simple correlation is presented to describe the onset of the degradation at long times and under normal service conditions. Both approaches give similar results and confirm the good thermal stability of PEEK matrix/carbon fiber composites at high service temperatures.