Poly(aryl ether ether ketone) (PEEK) is a high-temperature engineering thermoplastic used in the aerospace industry. To assess its suitability for nuclear industry applications, the effects of mixed field radiation from a nuclear reactor were investigated on two industrial semicrystalline PEEK grades (VICTREX 150P and 450P). Specimens were first processed on an ENGEL55 injection molder and then irradiated in the pool of a SLOWPOKE-2 nuclear research reactor. Specimens were thus exposed to a mixed field of radiation including gamma, electrons, protons, and neutrons, for exposures resulting in doses ranging from 0.15 to 15 MGy. Irradiated samples were characterized by viscosity measurements in the melt, differential scanning calorimetry (DSC), and density measurements. Melt viscosity of irradiated samples proved to be significant in detecting early molecular weight changes in PEEK and helped explain the interaction of competing chain scission and crosslinking reactions at various stages of the radiolysis. All results pointed toward the following transformation of the resin with increased dosage: chain scission of the tie-molecules between the amorphous and crystalline phases, crosslinking in. the amorphous phase, followed by a general degradation of the molecular weight dominated by chain-scission.