The production of thermoplastic composites (TPCs) requires multiple processing steps. These processing steps involve (repeated) heating of the composite above the melting temperature of the thermoplastic matrix. Prolonged heating at elevated temperatures might cause thermal degradation of the polymer, which causes structural changes in the polymer. These changes involve cross-linking and/or branching of the polymer chains which may influence the processability and final performance of the composite. TPC products are typically implemented in the aerospace and automobile industry, warranting the safety of installed components is of key importance. In this research, the thermal stability of C/PEEK has been analyzed by differential scanning calorimetry. The crystallization behavior was used as an indicator for thermal degradation. The DSC analysis shows that the ability of the polymer to crystallize is reduced after thermal conditioning. Prolonged heating results in reduction of crystallinity and retardation of crystallization kinetics. It is shown that the changes in crystallization kinetics can be described using a kinetic model. The results can be used to study and predict polymer degradation and its effect on the crystallization behavior in thermoplastic composites during processing.