Poly-Aryl-Ether-Ketones (PAEK) matrices of different composite preforms were evaluated to assess the influence of the preform manufacturing on the matrix integrity. Yarns, films, and impregnation suspensions were character- ized by gel permeation chromatography, Fourier transform infrared spectros- copy, thermogravimetric analysis, differential scanning calorimetry, and rheometry. As described in literature, changes in the matrix properties at mol- ten state are observed for all materials. The degradation process is related to a crosslinking process that results in an increase of viscosity and a decrease of crystallization temperature. Differences in degradation behavior were observed depending on the type of preform. Film and yarn extrusion has a limited effect on PAEK behavior, apart from a slight shift in molecular weight, without sig- nificant effect on degradation kinetics. However, the organic products that are used for the preparation of semifinished composite products, that is, yarn sizing or suspension additives, have a detrimental effect on stability. The deg- radation of additives considerably increases the degradation kinetics of the PAEK matrices, and by modifying the thermo-oxidation mechanisms, the advantages of using complex preforms for consolidation can be annihilated.